Musannaf Ibn Abi Shayba 3:3665

Yazid ibn Harun narrated to us, from Hisham, from al-Hasan, who said: If a man says in the prayer: "Indeed, Allah and His angels send blessings upon the Prophet. O you who have believed, ask [Allah to] send blessing upon him and ask [Allah to] send peace upon him," Then he should send blessings upon him. He said: And Ibn Sirin said: When they used to recite the Quran, they would not mix into it what was not from it, and they would proceed as they were.