Musannaf Ibn Abi Shaybah

Hazrat Abdul Kareem says that if Hazrat Saeed bin Musayyib and Hazrat Abu Ubaidah became doubtful about the number of units (rak‘ahs) in prayer and could not determine whether they had prayed three or four, both of them would perform the prostration of forgetfulness (sujood as-sahw) before concluding the prayer with salaam.