Musannaf Ibn Abi Shayba 3:5783

Abu al-Ahwas narrated to us, from Abu Ishaq, from Ibn Abi Layla, who said: Salman went out with thirteen men of the companions of the Messenger of Allah (may Allah bless him and grant him peace) on a military expedition, and Salman was the oldest of them. When the time for prayer arrived, they said to him, "Step forward, O Abu Abdullah." He said, "I am not one to step forward while you are the Arabs, and from among you is the Prophet (may Allah bless him and grant him peace), so let one of you step forward." So one of the people stepped forward and led them in prayer for four rak'ahs. When we finished the prayer, Salman said, "And what is with the four? Two rak'ahs, half of the four, would have sufficed us."