Musnad Abu Ya'la al-Mawsili 166:16

Abu Salamah ibn al-Sabbak narrated to us, [saying] Makhlad narrated to us, from Ibn Jurayj, from Abdullah ibn Musafi’, from Uqbah ibn Muhammad ibn al-Harith, from Abdullah ibn Ja’far, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever is in doubt regarding his prayer, let him prostrate two prostrations after he performs the taslim."