Musannaf Ibn Abi Shayba 21:128

Ahmad ibn Abd al-Malik ibn Waqid narrated to us, al-Harith ibn Murrah informed us, Yazid al-Raqashi narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When it is the time of the Adhan, the gates of the heaven are opened and the supplication is answered. And when it is the time of the Iqamah, a supplication is not rejected."