Musnad Ahmad 30:2745

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Warqa', from 'Amr ibn Dinar, from 'Ata' ibn Yasar, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, that he said: "When the prayer is established, there is no prayer except the obligatory one."