Musnad Ahmad 30:2367

Isma'il narrated to us, he said: Yunus—meaning Ibn 'Ubayd—informed us, from al-Hasan, from Anas ibn Hakim al-Dabbi, that he feared the time of Ziyad, or Ibn Ziyad, so he came to Medina and met Abu Hurayrah. He asked me about my lineage, and I told him my lineage. He said: "O young man, shall I not narrate to you a hadith that perhaps Allah will benefit you with?" I said: "Yes, may Allah have mercy on you." He said: "Indeed, among the first things for which people will be held accountable on the Day of Resurrection is the prayer." He said: "Our Lord, Mighty and Majestic is He, will say to His angels—and He is more knowing—: 'Look into the prayer of My servant; did he complete it or diminish it?' If it was complete, it is recorded for him as complete. And if he diminished anything from it, He will say: 'Look, does My servant have any voluntary [prayers]?' If he has voluntary [prayers], He will say: 'Complete the obligatory prayer of My servant from his voluntary [prayers].'" "Then the deeds will be taken in that manner." Yunus said: "And I think he mentioned the Prophet, may Allah bless him and grant him peace."