Musnad Ahmad 30:1266

Uthman ibn Umar narrated to us, Usamah narrated to us, from Sa’id al-Maqburi, from Abu Hurairah, that a man came to the Prophet, may Allah bless him and grant him peace, wanting to travel, to bid him farewell. The Messenger of Allah, may Allah bless him and grant him peace, said to him: "I advise you to fear Allah, and to say 'Allahu Akbar' (Allah is the Greatest) upon every elevated place." When he turned away, he said: "O Allah, fold for him the distance, and make the travel easy for him."