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

And from him, from Ali bin Ibrahim, from his father, from Hammad, from Hariz, from Muhammad bin Muslim, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who is praying and does not know whether he prayed one rak‘ah or two. Imam عليه السلام said: "He should start over until he is certain that he has completed (the prayer), and this applies in Jumu‘ah, in Maghrib, and in prayer during travel."