Musannaf Ibn Abi Shayba 34:315

Muslama narrated to us, Abu Ali al-Hasan ibn Mansur al-Baghdadi narrated to us, Abu Salama—meaning Ibn Isma'il al-Minqari—narrated to us, Aban—meaning Ibn Yazid al-'Attar—narrated to us, he said: Qatada informed us, from al-Hasan, from Anas ibn Hakim, from Abu Hurayra, that the Prophet, may God 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 it is sound, then he has succeeded and prospered; and if it is corrupt, then he has failed and lost."