Musnad Abu Ya'la al-Mawsili 166:6

Musa ibn Muhammad narrated to us, Rawh ibn 'Ubadah narrated to us, Ibn Jurayj narrated to us, he said: 'Abdullah ibn Musafi' narrated to 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 (may Allah bless him and grant him peace) said: "Whoever is in doubt regarding his prayer, let him perform two prostrations while he is sitting."