Musnad Ahmad 941:24

Harun ibn Ma'ruf narrated to us, Abdullah ibn Wahb narrated to us, 'Amr ibn al-Harith informed me, that Muhammad ibn al-Munkadir narrated to him, that he was informed by one who heard Usamah ibn Zayd saying: "The Messenger of Allah, peace and blessings of Allah be upon him, combined the Maghrib and Isha prayers at al-Muzdalifah."