Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 965
Narated By Abdullah ibn Mas'ud : Alqamah said that Abdullah ibn Mas'ud caught hold of his hand saying that the Apostle of Allah (pbuh) caught hold of his (Ibn Mas'ud's) hand and taught him the tashahhud during prayer.
He then narrated the (well known) tradition (of tashahhud). This version adds: When you say this or finish this, then you have completed your prayer. If you want to stand up, then stand, and if you want to remain sitting, then remain sitting.