Musannaf Ibn Abi Shayba 34:276

Affan narrated to us, Hammad narrated to us, he said: Al-Azraq ibn Qays informed me, from Yahya ibn Ya'mar, from a man among the companions of the Prophet, may Allah bless him and grant him peace, that the Prophet, may Allah bless him and grant him peace, 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 it was not complete, He will say: 'Look, do you find any voluntary acts for My servant?' Then complete with them what he neglected of his obligatory duties. Then the Zakat, then the deeds will be taken according to that."