Musnad Abu Ya'la al-Mawsili 166:14

Muhammad ibn Isma'il ibn Abi Samina narrated to us, Rawh narrated to us, Ibn Jurayj narrated to us, from 'Abdullah ibn Musafi', from Mus'ab ibn Shaybah, from 'Uqbah ibn Muhammad ibn al-Harith, from 'Abdullah ibn Ja'far, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you doubts in his prayer and does not know how much he has prayed, then let him prostrate two prostrations when he finishes his prayer."