Musnad Ahmad 30:3567

Rawh narrated to us, Zakariya ibn Ishaq narrated to us, Amr ibn Dinar narrated to us, he said: I heard Ata ibn Yasar saying, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, that he said: "When the prayer is established, there is no prayer except the obligatory one."