Musnad Abu Ya'la al-Mawsili 156:2

Shayban narrated to us, Abu al-Ashhab narrated to us, from al-Hasan, who said: Abu Hurayrah met a man in Medina and said to him, "It is as if you are not from the people of this city." He said, "Yes." He said, "Shall I not narrate to you a hadith I heard from the Messenger of Allah, may Allah bless him and grant him peace, that perhaps Allah will benefit you with?" He said, "Yes." He said, "I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: 'The first thing the son of Adam will be held accountable for is his prayer. Allah will say to His angels: Look into the prayer of My servant. If they find it complete, it is recorded for him as complete. And if they find that anything is missing from it, He will say: Look, do you find any voluntary [prayers] for him?'" He said, "Then his prayer is completed from his voluntary [prayers], and the deeds are taken according to that."