Musnad Ahmad 1167:44

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us, from Abu al-Aswad, from 'Urwah, from Asma bint Abi Bakr, she said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, while he was reciting while praying towards the corner [the Ka'bah] before he was commanded to declare what he was commanded to declare, and the polytheists were listening: 'So which of the favors of your Lord would you deny?'"