It is narrated from Sayyiduna Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When any one of you performs prayer and does not know how many units he has prayed, whether three or four, then he should perform one (additional) unit, perfecting its bowing and prostration, and (at the end) perform two prostrations." Muhammad bin Yahya states that I have also seen this narration as mawquf (stopped at the Companion) in another place in the book of Ayyub. Imam Abu Bakr (may Allah have mercy on him) says that Umar bin Muhammad is the son of Zaid bin Abdullah bin Umar bin Khattab (may Allah be pleased with him), and he is the elder brother of Asim and Waqid. He says that I heard Ahmad bin Sa'id al-Dari (may Allah have mercy on him) say that Asim, Umar, Zaid, Waqid, Abu Bakr, and Farqad are all brothers. And Asim is the son of Muhammad bin Zaid bin Abdullah bin Umar bin Khattab (may Allah be pleased with him). Imam Abu Bakr (may Allah have mercy on him) says that Imam Darimi mentioned this to us after his narration.