Musnad Ahmad 32:641

Aswad and Husayn ibn Muhammad narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from Burayd ibn Abi Maryam, from Anas, 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."