Musnad Ahmad 26:525

Muhammad ibn Salamah narrated to us, from Khasif, from Abu Ubaydah, from his father Abdullah ibn Mas’ud, from the Prophet (peace and blessings of Allah be upon him), who said: "If you are in prayer and you are in doubt between three and four, and your strongest inclination is toward four, then recite the Tashahhud, then prostrate twice while you are sitting before you say the Taslim. Then recite the Tashahhud again, and then say the Taslim."