Musnad Ahmad 941:19

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Hisham ibn 'Urwah narrated to me, from his father, from Usamah ibn Zayd, who said: I was riding behind the Messenger of Allah, may Allah bless him and grant him peace, on the evening of 'Arafah. He said: When the sun set, the Messenger of Allah, may Allah bless him and grant him peace, departed. When he heard the clamor of the people behind him, he said: "Gently, O people! Upon you is tranquility, for righteousness is not in rushing." He said: Whenever the people crowded upon the Messenger of Allah, may Allah bless him and grant him peace, he would quicken his pace, and when he found an opening, he would trot, until he reached Muzdalifah, where he combined the two prayers: Maghrib and the final 'Isha'.