Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When any of you prays and does not know how many rak'ahs he has prayed, three or four, let him pray one (more) rak'ah, performing its bowing and prostration well, then make two prostrations (of forgetfulness)." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.