Musnad Ahmad 30:3035

Waki’ narrated to us, from Usamah ibn Zayd—meaning al-Laythi—from al-Maqburi, that he heard it from Abu Hurayrah, who said: A man came to the Prophet, peace and blessings of Allah be upon him, intending to travel, and said: "O Messenger of Allah, advise me." He said: "I advise you to fear Allah (taqwa), and to say 'Allahu Akbar' (God is the Greatest) upon every elevated place." When the man departed, he said: "O Allah, fold the earth for him and make the journey easy for him."