Musannaf Ibn Abi Shayba 3:2030

Ibn Fudayl narrated to us, from Khusayf, from Abu Ubaydah, from Abdullah, who said: If one of you is in doubt regarding his prayer, let him seek the most likely of his thoughts and build upon it. If his most likely thought is that he prayed three [rak'ahs], let him perform one rak'ah and prostrate twice. And if his thought is four, let him prostrate twice.