Bismillah

Many scholars like Imam Bukharee rahmatulillah alay and Imam Muslim rahmatulillah alay have clarified the dangers of using da’eef (weak) ahadeeth.  Of the scholars that have used weak ahadeeth, they have done so with very strict conditions.

Ibn Hajar al-Askalani rahmatullilah alay, the father of the scholars of hadeeth, has said da’eef ahadeeth are only to be used when the following conditions are met:

  1. It is well accepted amongst the scholars of hadeeth that the weakness is only minor.
  2. The hadeeth is only used in light of the subject matter being well-established in other more authentic ahadeeth or the Qur’an
  3. The user of the da’eef hadeeth should not believe that it is really an act of the Prophet sallalahu alayhi wasalam because then it is possible we would be attributing acts to the Prophet sallalahu alayhi wasalam that he may not have done. If we believed that it was truly the saying of the Prophet sallalahu alayhi wasalam we would under threat of his saying: “Whoever lies upon me intentionally then let him be sure of his place in the hellfire”
  4. We should not call people to any act based on da’eef ahadeeth
  5. A weak hadeeth can only be used to encourage good deeds which are already well-established in other more authentic ahadeeth or the Qur’an
  6. We cannot use da’eef ahadeeth to establish Islamic belief, creed, or matters of halal and haraam.

Source

Shakir, Abdur-Ra’uf.  “Usool us-Sunnah’ by Imam Ahmed ibn al-Hanbal: Lecture 1″ http://www.islamlecture.com/usoolussunnah.htm