450
Musnad AhmedIt was narrated that "Uthman bin "Affan (رضي الله عنه) said: A man came to the Prophet (ﷺ) and said:
O Messenger of Allah (ﷺ), I prayed but I do not know whether I did an even number [of rak"ahs] or an odd number. The Messenger of Allah (ﷺ) said: "Beware lest the Shaitan toy with you in your prayer. Whoever among you prays and does not know whether he did an even number [of rak"ahs] or an odd number, let him prostrate twice, for that will complete his prayer."
Reference: Musnad Ahmad 450
Arabic/English book : Book 4, Hadith 450
Arabic/English book : Book 4, Hadith 450