Sayyiduna Mu'adh bin Anas (may Allah be pleased with him) relates that the Prophet (peace and blessings be upon him) said: "When you hear the mu'adhdhin making the call (tathwib) for prayer (that is, giving the adhan or saying the iqamah), say words like those he is saying."
Imam Ahmad and Imam al-Tabarani in al-Mu'jam al-Kabir recorded this narration. Its chain contains a narrator, Ibn Lahi'ah, in whom there is weakness.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1866