Musnad Abu Ya'la al-Mawsili 139:14

Ibrahim ibn al-Hajjaj narrated to us, Sahl ibn Ziyad narrated to us, from al-Taymi, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When the call to prayer is made, the gates of the heaven are opened, and the supplication is answered."