Musnad Ahmad 38:16

Yahya ibn Sa'id narrated to us, from Sufyan, who said: Salamah ibn Kuhayl narrated to us, from 'Abdullah ibn 'Abd al-Rahman ibn Abza, from his father, who said: When the Messenger of Allah (may Allah bless him and grant him peace) would reach the morning, he would 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 Ibrahim, inclining toward truth, a Muslim, and he was not of the polytheists."