Musnad Ahmad 1077:4

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn Abi Sa'ir, from Jabir ibn 'Abd Allah, who said: When the day of Uhud arrived, the Prophet, may Allah bless him and grant him peace, looked over the martyrs who were killed that day and said: "Wrap them in their blood, for I am a witness over them." So, two or three men would be buried in a single grave, and he would ask: "Which of them was more learned in the Quran?" Then they would present him first. Jabir said: My father and my uncle were buried on that day in one grave.