Musnad Ahmad 38:13

'Abd al-Rahman narrated to us, from Shu'bah, from Salamah ibn Kuhayl, from Dharr, from Sa'id ibn 'Abd al-Rahman ibn Abza, from his father, that the Messenger of Allah (may Allah bless him and grant him peace) used to say: "We have reached the morning upon the natural disposition (fitrah) of Islam, the word of sincerity, the religion of our Prophet Muhammad (may Allah bless him and grant him peace), and the creed of our father Abraham, inclining toward truth, and he was not of the polytheists."