Musannaf Ibn Abi Shayba 3:4717

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Yunus bin Ubayd informed us, he said: Abu Uthman al-Yashkuri narrated to me, he said: Anas bin Malik passed by us while we had already prayed the morning prayer, and with him was a group of people. He ordered a man from among them, so he performed the adhan. Then they prayed two rak'ahs before the dawn (prayer). He said: Then they ordered him, so he performed the iqamah. Then he stepped forward and led them in prayer.