Musnad Ahmad 4049

It was narrated from Abu ‘Ubaidah, from his father 'Abdullah bin Mas'ood, that The Prophet (ﷺ) said: “If you are praying and you are not sure whether you did three or four (rak'ahs), but you think it most likely that you did four, then say the tashahhud, then do two prostrations when you are sitting, before you say the salam. Then say the tashahhud again, then say the salam.”