Sa‘d, from Abu Ja‘far, from Muhammad ibn Khalid al-Barqi, from al-Hasan ibn Ali ibn Faddal, from Marwan ibn Muslim, from Ubayd ibn Zurarah, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who doubted and did not know whether he had prostrated twice or once, so he performed another prostration, and then became certain that he had added a prostration. Imam عليه السلام said: "No, by Allah سبحانه وتعالى, an extra prostration does not invalidate the prayer." And Imam عليه السلام said: "He does not repeat his prayer because of a prostration, but he does repeat it because of a rak‘ah."