Al-Kāfi

3. It is a narration from him (narrator of previous Hadith) by Ahmad ibn Muhammad from Ahmad ibn Muhammad ibn abu Nasr and Ali ibn Muhammad from Sahl ibn Ziyad from Ahmad ibn Muhammad ibn abu Nasr who has said the following: “I once asked abu al-Hassan (a.s.), about a man who performs one Rak’at of Salah (prayer), then he remembers in the second Rak’at in Ruku’ (bowing down on one’s knees) position that he has omitted one Sajdah (prostration) of the first Rak’at. He said, ‘Abu al-Hassan (a.s.), would say that if you omit Sajdah (prostration) in the first Rak’at and do not know if it is one or two; perform Salah (prayer) again until it is correct to say that two Sajdah (prostrations) are performed.’”