Musannaf Ibn Abi Shayba 34:236

Yazid narrated to us, Sufyan bin Husayn informed us, from Ali bin Zayd, from Anas bin 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, may Allah bless him and grant him peace, saying: 'The first thing the servant will be held accountable for on the Day of Resurrection is the obligatory prayer.'"