What is the difference among Hasan Lighairihi, weak and fabricated Ahadith? Can we act on weak Hadith?
Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Hadith Hasan Lighairihi (Hasan due to others) means, a number of similar weak Ahadith on a particular issue can be raised to the degree of Hasan (agreeable) if the weakness found in their reporters is of a mild nature. However, in case the weakness is severe (e.g., the reporter is accused of lying or the Hadith is itself irregular), such very weak Ahadith will not support each other and will remain weak. Hadith Hasan Lighairihi is an acceptable Hadith so, the scholars are agreed that it should be accepted and should be acted according to it. This is the opinion of the majority of Muslim scholars.
The Da'ef (weak) Hadith is that which fails to reach the status of Hasan. The Da'ef Hadith is not accepted and not put into actions if the matter is related to Halal, Haram, and Wajibat since such rulings cannot be proved through a weak Hadith.
However, many scholars are of the opinion that a weak Hadith could be accepted by some conditions if it is related to the subject of virtues of some goods deeds.
Maudu, means fabricated, forged. It is strongly forbidden to narrate such Ahadith let alone acting according to it. Some scholars are of the opinion that whoever intentionally fabricates a Hadith becomes Kafir because he introduces something in Shariah, which is not from it.
Allah knows best.