Musannaf Ibn Abi Shayba 2:240

Abu Bakr narrated to us, he said: Abu Abd al-Rahman al-Muqri’ narrated to us, from Sa’id ibn Abi Ayyub, he said: Ka’b ibn Alqamah narrated to me, from Abd al-Rahman ibn Jubayr, from Abdullah ibn Amr, he said: The Prophet (peace and blessings of Allah be upon him) said: "When you hear the caller to prayer, say as he says."