Musnad Ahmad 30:2596

Waki’ narrated to us, Usamah bin Zaid narrated to us, from Sa’id al-Maqburi, from Abu Hurairah, who said: A man came to the Prophet, may Allah bless him and grant him peace, wanting to go on a journey, and said: "O Messenger of Allah, advise me." He said: "I advise you to fear Allah, and to say 'Allahu Akbar' (Allah is the Greatest) upon every elevated place." When he turned to leave, he said: "O Allah, fold the earth for him, and make the journey easy for him."