Musannaf Ibn Abi Shayba 34:22

Ghundar narrated to us, from Shu'bah, from Habib ibn al-Shahid, from Ibn Abi Najih, from Mujahid, who said: The first to speak aloud, or the first to announce the taslim (salutation) in the prayer, was Umar ibn al-Khattab.