Musnad Abu Ya'la al-Mawsili 108:184

Ishaq ibn Abi Isra'il narrated to us, Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from Ibn Abi Sa'ir, from Jabir ibn Abdullah, who said: When the day of Uhud arrived, the Prophet, may Allah bless him and grant him peace, looked over the martyrs who were martyred on 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. He would ask which of them was more learned in the Quran, and he would place him forward. Jabir said: So my father and my uncle were buried on that day in one grave.