Musnad Ahmad 26:650

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sulayman, who said: I heard Abu Wa'il narrating from 'Abdullah, from the Prophet, peace and blessings of Allah be upon him, that he said: "Indeed, the first thing that will be judged between the servants [on the Day of Resurrection] concerns the blood."