من لا يحضره الفقيه, volume 1, page 348

Hadith.1013 - Ubayd ibn Zurarah asked Imam Jafar ibn Muhammad Al-Sadiq (as) about a man who prays the Fajr prayer, offering one rak'ah, then makes the tashahhud (testification), turns away (ends the prayer), and leaves. After some time, he remembers that he only prayed one rak'ah. He asked what should he do. Imam (as) replied: "He should add another rak'ah to it”.

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.

Hadith.1014 - Abu Kahmas asked Imam Jafar ibn Muhammad Al-Sadiq (as) about the two rak'ahs of prayer when, after sitting in the tashahhud position, he says: "As-Salamu Alayka Ayyuha An-Nabi wa Rahmatullahi wa Barakatuh" (Peace be upon you, O’ Prophet, and the mercy of Allah (swt) and His blessings). Is this considered the ending (of the prayer)? Imam (as) replied: "No, it is not the ending. But when you say 'As-Salamu Alayna wa Ala Ibadillah As-Saliheen' (Peace be upon us and upon the righteous servants of Allah (swt)), that is the ending."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.