Musnad Ahmad 26:526

Muhammad ibn Fudayl narrated to us, Khasif narrated to us, Abu Ubaydah ibn Abdullah narrated to me, from Abdullah ibn Mas’ud, who said: "If you are in doubt during your prayer while you are sitting, and you do not know whether you have prayed three [rak'ahs] or four, then if your strongest assumption is that you have prayed three, stand up and perform a rak'ah, then perform the taslim, then prostrate twice, then recite the tashahhud, then perform the taslim. And if your strongest assumption is that you have prayed four, perform the taslim, then prostrate twice, then recite the tashahhud, then perform the taslim."