Musnad Ahmed: Hadith 4050

If you are not sure about your prayer, when you are sitting, and you do not know whether you prayed three or four, but you think it most likely that you prayed three, then get up and do one rak'ah, then say the salam, then prostrate twice, then say the tashahhud, then say the salam. If you think it most likely that you prayed four, then say the salam, then prostrate twice, then say the tashahhud, then say the salam.