Musnad Ahmad 1153:1503

Isma'il ibn 'Umar narrated to us, he said: Yunus ibn Abi Ishaq narrated to us, from Abu Ishaq, from al-Aswad ibn Yazid, he said: I said to 'Aishah, the Mother of the Believers: "At what time do you perform the Witr prayer?" She said: "I do not perform the Witr until they give the Adhan, and they do not give the Adhan until the dawn breaks." She said: "The Messenger of Allah, peace and blessings of Allah be upon him, had two callers: Bilal and 'Amr ibn Umm Maktum. The Messenger of Allah, peace and blessings of Allah be upon him, said: 'When 'Amr gives the Adhan, then eat and drink, for he is a man who is blind. And when Bilal gives the Adhan, then restrain yourselves (from eating and drinking), for Bilal does not give the Adhan—so he said—until it becomes morning.'"