Musnad Ahmad 1153:843

Ibrahim ibn Ishaq narrated to us, he said: Ibn al-Mubarak narrated to us, from al-Awza'i and Ma'mar, from al-Zuhri, from 'Urwah, from 'Aishah, she said: "When the Mu'adhdhin would fall silent from the morning prayer, he [the Prophet] would pray two light rak'ahs," meaning the Prophet, peace and blessings of Allah be upon him.