It is narrated from Sayyidah Aishah (may Allah be pleased with her) that when the Messenger of Allah (peace and blessings be upon him) heard the mu'adhdhin bearing witness, he (peace and blessings be upon him) would say: "And I too, and I too (bear witness)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1929