What is the ruling on acting upon a weak hadith, and what are the conditions for acting upon it?

Working with a weak (da'if) hadith (definition) is permissible under certain conditions: that it pertains to the virtues of deeds, that it falls under an established principle that is acted upon, that it is not severely weak, and that its authenticity is not believed with certainty, but rather that it is considered a precautionary measure. What scholars mean by "working with it" is that the action itself is established as being loved or disliked by Allah through a text or consensus. So, if a hadith is narrated regarding the virtue of a recommended act or the dispraise of a disliked act, its narration and acting upon it are permissible in the sense of hoping for reward or fearing punishment, not as establishing its recommendability. However, if weak hadiths about virtues include estimations and specific delimitations, then it is not permissible to act upon them unless that specific description is established by legal evidence. The intent, then, is to narrate it and act upon it for encouragement (targhib) and deterrence (tarhib), not for establishing its recommendability, and believing in its legal obligation depends on legal evidence.