Musnad Abu Ya'la al-Mawsili 129:131

Narrated to us Zuhayr, narrated to us Yazid ibn Harun, informed us Humayd, from Anas, that the Prophet (peace and blessings of Allah be upon him) used to like for the Emigrants (Muhajirun) and the Helpers (Ansar) to be directly behind him in prayer, so that they might learn from him.