Musnad Ahmad 1060:75

Bahz narrated to us, Shu'bah narrated to us, 'Adi ibn Thabit narrated to us, from 'Abdullah ibn Yazid, from Abu Ayyub al-Ansari, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, combined the Maghrib and 'Isha prayers at Jam' (Muzdalifah)."