Musnad Ahmad 20:7

Rawh narrated to us, Ibn Jurayj narrated to us, Abdullah ibn Musafi' informed me that Mus'ab ibn Shaybah informed him, from Uqbah ibn Muhammad ibn al-Harith—and al-Hajjaj said: Utbah ibn Muhammad ibn al-Harith—from Abdullah ibn Ja'far, from the Prophet, may Allah bless him and grant him peace, who said: "Whoever is in doubt regarding his prayer, let him prostrate two prostrations while he is sitting."