Musnad Ahmad 30:784

Yazid narrated to us, Sufyan—meaning Ibn Husayn—informed us, from Ali ibn Zayd, from Anas ibn Hakim al-Dabbi, who said: Abu Hurayrah said to me: "When you come to the people of your city, inform them that I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The first thing for which a servant will be brought to account on the Day of Resurrection is his obligatory prayer. If it is sound'—and Yazid said once: 'If he completed it'—'then it is well; otherwise, it will be supplemented from his voluntary prayers. Then the rest of the obligatory deeds will be treated in the same manner.'"