Musannaf Ibn Abi Shayba 3:3870

Mu'tamir (Mu'ammar) narrated to us, from 'Awf, who said: A man from the people of the desert informed me. He said: A tax collector (musaddiq) from Medina came to us during the nights of Mu'awiyah. While he was at a water source of ours one day, the time for prayer arrived. There were two mosques of the desert people at the water source. He asked, "Which of the two was built first?" It was said to him, "This one." So he headed toward it.