Musnad Ahmad 38:9

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