Musannaf Ibn Abi Shayba 3:2379

Marwan ibn Mu'awiyah narrated to us, from 'Awf, from Abu Raja', from 'Imran ibn Husayn, who said: We traveled with the Messenger of Allah (may Allah bless him and grant him peace) on a journey, and we traveled through the night until, when it was the end of the night, we fell into that sleep—and there is no sleep sweeter to a traveler than it. Nothing awakened us except the heat of the sun. 'Umar began to say "Allahu Akbar." When he awoke, the people complained to him about what had befallen them. He said, "There is no harm." He said: So they departed and traveled a short distance, then he dismounted and the call to prayer was made, and he led the people in prayer.