Musannaf Ibn Abi Shayba 3:6331

Abu Bakr narrated to us, he said: Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, regarding the man who forgets the tashahhud until he exits his prayer. He said: "If he has exited it, then his prayer is complete, and if he has not exited it, he should perform the tashahhud." He said: "Exiting, according to him, was that he speaks, or enters into another prayer, or turns his back to the qibla."