It is narrated from Sayyiduna Mu'adh bin Anas al-Juhani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When you hear the mu'adhdhin calling to the prayer, say just as he says."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17571