Musannaf Ibn Abi Shayba 3:2028

Jarir narrated to us, from Abd al-Hamid, from Mansur, from al-Hakam, who said: Ali said: "If one is in doubt regarding an increase or a decrease [in the number of rak'ahs], let him pray one [additional] rak'ah, for Allah does not punish for an increase in the prayer. If it was complete, it is for him, and if it was an increase, it is for him."