Musannaf Ibn Abi Shayba 3:617

Mu'adh ibn Mu'adh narrated to us, from Habib ibn al-Shahid, who said: Muhammad was asked about the Tashahhud, and he said: "The pure prayers (al-salawat al-tayyibat)." He said: Then he said, "Ibn Abbas used to add 'the blessings' (al-barakat) to it."