Musnad Abu Ya'la al-Mawsili 151:36

Muhammad ibn Abdullah ibn Numayr narrated to us, Hafs narrated to us, from Mis'ar, from Mansur, from Ibrahim, from Alqamah, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you is in doubt, let him seek what is correct and perform two prostrations."