Musnad Ahmad 32:1724

Husayn ibn Muhammad narrated to us, Isra'il narrated to us, from Abu Ishaq, from Burayd ibn Abi Maryam, from Anas ibn Malik, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, the supplication is not rejected between the Adhan and the Iqamah, so supplicate."