Musnad Abu Ya'la al-Mawsili 152:346

Qasim ibn Abi Ubayd narrated to us, Ishaq al-Azraq narrated to us, Sharik narrated to us, from Asim, from Abu Wa'il, from Abdullah, from the Prophet (peace and blessings of Allah be upon him), who said: "The first thing for which the servant will be held accountable is the prayer. And the first thing that will be judged between the people on the Day of Resurrection is [regarding] the blood."