It is narrated from Sayyiduna Uthman (may Allah be pleased with him) that a man came to the service of the Prophet (peace and blessings be upon him) and said, "O Messenger of Allah! I was performing prayer, and I could not tell whether I had prayed an even or odd number of rak‘ahs." The Prophet (peace and blessings be upon him) said: "Protect yourself from letting Satan play with you during prayer. If any of you is praying and cannot tell whether it was an even or odd number, then he should perform two prostrations of forgetfulness (sujood as-sahw), for with these two, the prayer is completed."