Musnad Ahmad 1153:916

'Affan narrated to us, he said: 'Abd al-Wahid ibn Ziyad narrated to us, he said: 'Amr ibn Maymun ibn Mihran narrated to me, he said: My father informed me, he said: 'Aishah said: When the Messenger of Allah (peace and blessings of Allah be upon him) would hear the caller (to prayer), he would say: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah."