Musnad Ahmad 32:260

Waki’ narrated to us, Sufyan narrated to us, from Zayd al-‘Ammi, from Abu Iyas—meaning Mu’awiyah ibn Qurrah—from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Supplication is not rejected between the Adhan and the Iqamah."