Ahmad ibn Muhammad ibn Isa, from Ahmad ibn Muhammad ibn Abi Nasr, who said: I asked Abul Hasan عليه السلام, peace be upon him, about a man who performs two rak‘ahs of prayer, then remembers in the second rak‘ah while in the state of bowing that he had left out a prostration in the first. He (Imam al-Ridha عليه السلام) said: “Abul Hasan (Imam al-Kadhim عليه السلام), used to say: "If you left out a prostration in the first rak‘ah and did not know whether it was one or two, then you must start over until two prostrations are certain for you. But if it is in the third or fourth rak‘ah and you left a prostration after having already preserved the bowing (ruku‘), then you repeat the prostration."