Musnad Ahmad 20:12

Hajjaj narrated to us, Ibn Jurayj said: Abdullah ibn Musafi‘ informed me that Mus‘ab ibn Shaybah informed him, from ‘Uqbah ibn Muhammad ibn al-Harith, from Abdullah ibn Ja‘far, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever is in doubt regarding his prayer, let him prostrate two prostrations after he performs the taslim."