“I once said to abu ‘Abd Allah (a.s.), about a man who performs two Rak‘at of al-Witr, then stands up and forgets saying Tashahhud (testimonies of belief) until he performs Ruku‘ (bowing down on one’s knees), then remembers when he is in Ruku‘. He (the Imam) said, ‘He is required to sit down from Ruku‘, say Tashahhud, then stand up and complete.’ He (the narrator) has said that he asked, ‘Have you not said that in obligatory Salah (prayer) if one remembers after Ruku‘, one continues, then performs two Sajdah (prostration) because of mistake after ending Salah (prayer) in which one says Tashahhud (testimonies of belief)?’ He (the Imam) said, ‘Optional Salah (prayer) is not like obligatory one.’”