Al-Kāfi, vol. 3, no. 1378

1. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad ibn ‘Isa from ‘Uthman ibn ‘Isa from Sama‘ah ibn Mehran who has said the following: “Abu ‘Abd Allah (a.s.), has said, ‘If one knows the kind of mistake he has made, he completes it, and Sajdah (prostration) for mistake is not necessary for him. The Messenger of Allah performed al-Dhuhr Salah (prayer) with people as two Rak‘at, then he mistakenly said Salam (the phrase of offering greeting of peace). Dhu al-Shamalayn asked him, “O Messenger of Allah, is anything revealed about Salah (prayer)?” The Messenger of Allah asked, “Why is that?” He said that it is because you performed two Rak‘at. The Messenger of Allah then asked others, “Do you also say what he says?” They replied, “Yes, he is right.” The Messenger of Allah stood up, completed Salah (prayer) with them and performed two Sajdah (prostration) with them because of mistake? He (the narrator) has said that I then asked, ‘What do you say about the case of one who performs two Rak‘at and thinks that it is four, then says Salam (the phrase of offering greeting of peace) and moves away; then remembers after moving away that he has performed two Rak‘at? He (the Imam) said, ‘He performs it again from the beginning? He (the narrator) has said that he then asked, ‘Why did the Messenger of Allah not perform again? He only completed with them whatever remained of his Salah (prayer). He (the Imam) said, ‘It is because the Messenger of Allah had not left his place. If one does not leave his place, he completes what is deficient of his Salah (prayer), provided, he has secured the first two Rak‘at?”