Is it permissible to act upon a weak Hadith regarding the فضائل الأعمال (virtues of deeds), such as fasting and observing prayer on the night of the middle of Sha'ban and supplicating during ablution, or is that considered an innovation (bid'ah)?

Scholars have differed on acting upon weak hadith (definition). Some have forbidden it absolutely, while others have permitted it for virtuous deeds under certain conditions. Sheikh al-Islam Ibn Taymiyyah held that it is not permissible to rely in Sharia (definition) on weak hadiths that are neither sahih (definition) (authentic) nor hasan (definition) (good). However, he allowed their narration for virtuous deeds, so long as it is not known that they are fabricated, and provided that the deed itself is legislated by a Sharia proof. He clarified that the revival of the night of mid-Sha'ban and singling out its day for fasting, and making supplication at each limb during ablution, are not permissible to be substantiated by weak hadith due to the extreme weakness of the hadiths narrated concerning them, and because they do not fall under a general principle supported by authentic evidence.