May People Act According to a Weak Hadith?

First of all, we’d like to state that the science of Hadith is very deep and intricate. Therefore, only one who has the necessary qualifications and expertise in Hadith should engage in discussions of them. Scholars of Hadith should verify and check the authenticity of hadiths circulated by laymen and common people in order to explain what is acceptable and what is not.
Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America states: Generally, in Islamic law, only the authentic (sahih) and good (hasan) hadiths are used in deriving the rules. The weak (da`if) hadiths have no value for the purpose of Shari`ah. However, the scholars of hadith sometimes differ among themselves in the determination of whether some hadiths are weak or not. The scholars have sometimes used weak hadiths for moral and spiritual (fada’il) matters. It is important that when one uses a weak hadith for any reason, one should explain it to the people that this is weak hadith and that it is being used for this particular reason.
We must also keep in mind that according to the scholars of hadith (muhaddithin,) the hadiths are counted according to their chains of narrators (isnad) and not according to the texts (matn). Thus one text of a hadith is often found reported through many chains of reporters. Normally the authenticity or weakness of the hadith is checked, not on the basis of its text, but on the basis of its chain (isnad) only. The weakness in a hadith could be due to the lack of continuity in the chain of transmitters or due to the lack of integrity (`adalah) of its narrators. There are various categories of weak hadiths depending on what part of the chain of transmitters was interrupted and what lack of integrity is detected in a transmitter or transmitters of that particular hadith. It is also possible that a hadith is weak according to one narration (i.e. chain of transmission), but its text is found in another better and more sound narration. Thus the judgment in this matter requires a thorough knowledge of the hadith literature and methodology.