Musnad Ahmad 30:1191

Rawh narrated to us, Usamah ibn Zayd narrated to us, from Sa’id al-Maqburi, from Abu Hurayrah, 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 the man turned away, the Prophet, may Allah bless him and grant him peace, said: "O Allah, fold the earth for him, and make the journey easy for him."