Musnad Ahmad 26:663

Waki' and Humayd al-Ru'asi narrated to us, they said: al-A'mash narrated to us, from Abu Wa'il—Humayd said: Shaqiq ibn Salamah—from Abdullah, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The first thing that will be judged between people on the Day of Resurrection is [regarding] bloodshed."