Musnad Ahmad 3:73

Hashim narrated to us, Abu Ja'far al-Razi narrated to us, from 'Abd al-'Aziz ibn 'Umar, from Salih ibn Kaysan, from a man, from 'Uthman ibn 'Affan, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no Muslim who leaves his house, intending a journey or otherwise, and says when he leaves: 'In the name of Allah, I have believed in Allah, I have held fast to Allah, I have relied upon Allah, there is no power and no strength except by Allah,' but that he is provided with the good of that departure, and the evil of that departure is averted from him."