|Da'eef (Weak) is a type of Hadith that has a narrator who either doesn't have a strong memory or isn't virtuous. Weak (Daeef) Hadith is only reliable in excellence, not in rulings (permissibility or prohibition will not be proven for it. Yes, good deeds or the prominence of an individual can be).
The result of this is that a weak Hadith is not a lie, false or fabricated (contrary to the propaganda of Ghair-Muqallids). The Muhadditheen have kept its rank less than Sahih and Hasan merely for caution.
If a weak Hadith becomes a Hasan Hadith for some reason, it also becomes completely credible, with both excellence and rulings able to be proven from it.
A weak narration sometimes becomes Hasan in the following situations:
1. If it is reported by two or more chains of narration, even if all are weak. In other words, if a Hadith is reported through a few weak narrations, it becomes Hasan. [Mirqaat, Mauzuaat-e-Kabeer, Shaami, Introduction to Mishkaat by Shaikh Abdul-Haqq Radi ALLAHu Ta'ala Anho Risaala Usool-e-Hadith by Imam Jurjaani Radi ALLAHu Ta'ala Anho]
2. The practice of true Ulama also causes a weak Hadith to become Hasan. Thus, if the Learned Ulama-e-Deen begin to bring practice on a weak Hadith, it doesn't remain so, but becomes Hasan. It was for is reason that Imam Tirrnidhi Radi ALLAHu Ta'ala Anho once said, As this Hadith is Ghareeb or weak (Daeef), but the learned practice It."
This saying of Imam Tirmidhi Radi ALLAHu Ta'ala Anho doesn't mean that this Hadith is weak and unworthy of practice and that the Ulama of Islam have practiced on it out of stupidity or have become astray. No. It means that the Hadith is weak in regards to its narrators, but through the practice of Ulama, it has been strengthened.
3. Through the experience of the Ulama and inspiration (Kashf) of the Friends of Allah SubHanuhu wa Ta'ala (Auliya), a weak Hadith can become strong. Shaikh Muhayuddin Ibn Arabi Radi ALLAHu Ta'ala Anho knew of a Hadith,
'The person who recites the Kalima Tayyiba 70,000 times attains forgiveness."
Once, a young map said to him,
"I see my deceased mother in Jahannam."
At that point, Shaikh Ibn Arabi had already recited the Kaliina Tayyiba 70,000 times, so (in his heart) he passed on the recital of the Kalima Sharif to that young man's mother. The man smiled and said,
"I now see my mother in Jannat."
Shaikh Ibn Arabi Radi ALLAHu Ta'ala Anho later said,
"I understood this Hadith's correctness (Sehat) through the inspiration (Kashf) of this Wali." (Sahih-ulBihaari)
4. Qaasim Nanautwi has recorded this incident regarding Hadrat Junaid Baghdadi Radi ALLAHu Ta'ala Anho in his book, Tahzeer-un-Naas.
5. The weakness of the chains of narration (Isnaad) doesn't necessitate the text of the Hadith to also be weak. Thus, it is possible for a single Hadith to be weak in one chain, Hasan in a second and Sahih in a third. This is why Imam Tirmidhi Radi ALLAHu Ta'ala Anho once said regarding a single Hadith, "This Hadith is Hasan, Sahih and Ghareeb as well"
This saying of the Imam can only mean that this Hadith is reported through a few chains: Hasan in the first, Sahih in the second and Ghareeb in the third.
6. Also, the weakness doesn't negatively affect the Muhaddith or Mujtahid of former times. Therefore, if Imam Bukhari Radi ALLAHu Ta'ala Anho or Imam Tirmidhi Radi ALLAHu Ta'ala Anho attained a Hadith as weak (Daeef) due to a weak narrator being included in it, it is possible for Imam Abu Hanifa Radi ALLAHu Ta'ala Anho to have attained that same Hadith while it had a Sahih chain of narrators (i.e. in his time, that weak narrator was not a part of the chain of Hadith). Thus, it wouldn't be easy for a Wahabi to prove that a Hadith was attained by Imam Abu Hanifa Radi ALLAHu Ta'ala Anho as weak.
As a result, our Sunni Ulama should bear this in mind. When any Wahabi calls a Hadith "weak", stop him, request its reason of weakness and then clarify whether this was before or after Imam Abu Hanifa's Radi ALLAHu Ta'ala Anho time. Insha-Allah, this will silence them. Imam Abu Hanifa's Radi ALLAHu Ta'ala Anho time is extremely close to the Holy Prophet Sallallaho Alaihi wa Sallam's. At that time, very few Ahadith were weak. Imam Sahib was even a Taba'ee (one who has met at least one Companion of Rasoolullah Sallallaho Alaihi wa Sallam).
7. Jarah-e-Mubham (ambiguous impugnment) is not worthy of acceptance. In other words, the mere statement of one who contests a Hadith, especially Ibn Jauzi, etc. (saying "This Hadith is weak" or "That narrator is weak") is not credible unless they explain the reason of the Hadith's weakness or narrator's defect, because Imams differ in the reasons of weakness. Some deem something to be a defect while others do not. Tadlees, lrsaal, immaturity, being busy in Fiqh, etc. have been accepted as defects for a narrator. However, according to the Hanafi muzhab, none of these reasons qualify. [Noor al-Anwaar. Discussion on Ta'an ala al-Hadith]
8. When there is a clash between impingement (Jarh) and placing in order (Tadeel), Tadeel will be accepted, not Jarh. Meaning, if a Muhaddith (Commentator on Hadith) has called a certain narrator weak while another has called him strong (in a case where his transgression is seen in some events yet he is still labeled by some as pious and virtuous), he will be accepted as pious and his narration will not be considered weak, because piety is the original state ofa Mu'min.
9. It should be remembered that if a Hadith is not sound (Sahih), it doesn't mean it's weak. Thus, if a Muhaddith says about a certain Hadith, "It's not Sahih", it doesn't necessarily mean that the Hadith is considered weak (there is a possibility of it being classified as Hasan). There are several categories of Hadith between sound and weak (Sahih & Daeef).
10. The basis of Sahih Ahadith is not Muslim, Bukhari or the rest of the Sihah Sita (Six Famous Books of Hadith).
The latter's namesake doesn't mean that all the Ahadith in these books are Sahih whilst every narration in other books aren't.
Rather, it means that there are many Sahih Ahadith in them. Our Imaan is on the Prophet, not on Bukhari, Muslim, etc. Wherever the Hadith of the Holy Prophet Sallallaho Alaihi wa Sallam is found, it is a treasure for us irrespective of whether it is in Bukhari Sharif or not.
It's really surprising that Ghair-Muqallids label the Taqleed of Imam Abu Hanifa Radi ALLAHu Ta'ala Anho and other Imams as polytheism (shirk) yet blindly bring Imaan on Bukhari and Muslim as if they were making Taqleed of these books.
11. The accepting of a Hadith without any query by a Faqih Aalim is proof of it being strong. Thus, if any Mujtahid, Faqih Aalim accepts a weak Hadith, it becomes strong. Imam Waliuddin Tabrezi Radi ALLAHu Ta'ala Anho the compiler of Mishkaat, states, "When I ascribe the Hadith to these Commentators of Hadith (Muhadditheen), it is as if I have attributed it to the Holy Prophet Sallallaho Alaihi wa Sallam himself." (Introduction to Mishkaat)
From these principles, you have understood that none of the Ahadith which Imam Abu Hanifa Radi ALLAHu Ta'ala Anho has used as proof can be weak because the Ummah acts upon them and the Ulama and Fuqahaa have also accepted them. Every Hadith has been reported by several chains of narration.
12. If a conflict between the Hadith and Quran is seen the Hadith should be interpreted and given a meaning that causes conformity between the two and the clash to be lifted. Similarly, if the Ahadith are seemingly conflicting, it is necessary to give a meaning to them so that the conflict is erased and practice can be made on all. Examples of this follow,
AllahSubHanuhu wa Ta'ala states, "Read the amount of the Quran which is easy in namaaz," However, a Hadith states, "The Salaah of he who doesn't read Surah Fatiha in it is not done."
Visibly, the Hadith contradicts the ayat. For this reason, the Hadith's meaning is taken that without reciting Surah Fatiha, Salaah is not perfected. Qirat (recitation) is absolutely Fardh in namaaz and reciting Surah Fatiha is Waajib. The conflict is lifted and both the Quran and Hadith have now been acted upon.
The above Hadith also seems to go against the following ayat, "When the Quran is recited, listen to it attentively and remain silent." [Surah Aaraal, Verse 204]
Here, the Quran orders complete silence, while the muqtadi (person following the Imam in namaaz) is told to recite Surah Fatiha. Thus, we shall have to conclude that the Quran's command is absolute while the order of the Hadith concerns someone reading namaaz alone or is addressing the Imam who is leading the Salaah. The recitation of the Imam is sufficient for the muqtadi. This principle is very important.
Also, if any Hadith clashes with a Quranic verse or a stronger Hadith than itself and there is no way of conformity occurring, the Quranic ayat or superior Hadith will be given preference and the Hadith will be regarded as unacceptable for practice (being termed as either inapplicable (mansookh) or only for the uniqueness of the Holy Prophet Sallallaho Alaihi wa Sallam). There are examples of this.
13. A Hadith being proven to have weak narration is Qiyaamat for Ghair Muqallids because the basis of their muzhab is these narrations alone. If a narration becomes weak, their ruling is revoked. However, this does not affect Hanafis, etc. because our proofs are not these narrations but only the verdict of the Imam (the Hadith corroborates his verdict). Yes, the proof of the Imam is Quran and Hadith, but when Imam Abu Hanifa Radi ALLAHu Ta'ala Anho attained the Ahadith he issued verdicts from. They were all sound (i.e. their chains of narration were not those which are found in Bukhari and Muslim today).
Likewise, if the police throw a criminal in jail, their proof is the judgement of the judge, not the articles of the government's law. The articles are the proof of the judge or ruler. This is important to remember. Taqleed (i.e. following the Four Imams) is Allah’s SubHanuhu wa Ta'ala mercy and GhairMuqallids (i.e. those who reject it) are His punishment.
Almighty Allah SubHanuhu wa Ta'ala and His Beloved Rasool Sallallaho Alaihi wa Aalihi wa Sallam knows the best!