Musnad Ahmad 30:1805

Qutaybah narrated to us, Bakr ibn Mudar narrated to us, from 'Amarah ibn Ghaziyah, from Abu Salih, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Faith is sixty-four doors; the highest and loftiest of them is the saying: 'There is no god but Allah,' and the lowest of them is removing harm from the road."