Musnad Ahmad 298:1

Waki’ narrated to us, from Sulayman ibn al-Mughirah, from Humayd ibn Hilal, from Hisham ibn ‘Amir al-Ansari, who said: When the day of Uhud came, the people suffered wounds and severe hardship. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Dig, widen, and bury two or three in one grave." They said: "O Messenger of Allah, whom should we put forward?" He said: "The one among them who had memorized the most of the Quran."