Musnad Abu Ya'la al-Mawsili 159:131

Muhammad ibn al-Khattab narrated to us, Mu'ammal narrated to us, Hammad ibn Salamah narrated to us, 'Amr ibn Dinar narrated to us, from 'Ata' ibn Yasar, from Abu Hurayrah, that the Prophet (peace and blessings of Allah be upon him) said: "When the prayer is established, there is no prayer except the obligatory one."