Chapter on Rulings on Forgetfulness (Sahw) in Prayer and What Requires Repeating of Prayer

And from him, from Ali bin Ibrahim, from his father, and from Muhammad bin Isma‘il, from al-Fadl bin Shadhan, all from Hammad bin ‘Isa, from Hariz, from Zurarah, from one of them (Imams عليه السلام), peace be upon them both, who said: I said to Him عليه السلام: “A person does not know whether he is in the fourth rak‘ah or the second, but he is certain that he has completed two rak‘ahs.” Imam عليه السلام said: "He should pray two rak‘ahs and four prostrations while standing, reciting the Opening of the Book (Surah al-Fatiha), and perform the tashahhud, and nothing is upon him. And if he does not know whether he is in the third or the fourth, and he is certain he has completed three rak‘ahs, then he should stand and add one more, and nothing is upon him. One does not nullify certainty with doubt, and one should not insert doubt into certainty, nor mix the two together. Rather, doubt is nullified by certainty, and he completes the prayer based on what is certain and builds upon it, and doubt is not to be regarded in any situation."