Musnad Ahmad 30:3743

Azhar ibn al-Qasim narrated to us, Zakariya ibn Ishaq narrated to us, from ‘Amr ibn Dinar, from ‘Ata’ ibn Yasar, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When the prayer is called (iqamah), there is no prayer except the prescribed one."