Musnad Ahmad 874:1

'Affan narrated to us, Hammad bin Salamah narrated to us, al-Azraq bin Qays narrated to us, from Yahya bin Ya'mar, from a man among the companions of the Prophet, may Allah bless him and grant him peace, who said: "The first thing the servant will be held accountable for on the Day of Resurrection is his prayer. If he completed it, it is recorded for him as complete. And if he did not complete it, He will say: 'Look, do you find any voluntary acts for My servant?' Then complete what he neglected of his obligatory acts with them. Then the Zakat, then the deeds will be taken according to that."