Musannaf Ibn Abi Shayba 3:2038

Katheer ibn Hisham narrated to us, from Furat, from Abd al-Karim, from Sa’id ibn al-Musayyib and Abu Ubaydah, that when they were in their prayer and did not know whether they had prayed three [rak'ahs] or four, they would prostrate two prostrations before they performed the taslim.