What is the ruling on narrating a weak hadith and acting upon it?

Question:
If the hadith, considering all its chains of narration, does not reach the level of hasan li-ghayrihi and remains weak; is it not acted upon in matters of spiritual exhortation?

Answer:
No, it does not establish a sunnah, but it is used for encouragement, in the sense of encouraging what is already established. That is, if it concerns prayer and fasting, it encourages what is established.
Q: Is it permissible to act upon it?
The Shaykh: It falls under the category of encouragement, of inciting interest in something that is established. It does not establish a ruling itself; a weak hadith does not establish a ruling, but it encourages adherence to established rulings. For example, regarding the virtue of fasting in Ramadan, the virtue of fasting on the Day of 'Arafah, and the virtue of fasting on Mondays and Thursdays. These are established by authentic evidence, but when hadiths come that incite interest in them, it is in the sense of encouragement.