Musannaf Ibn Abi Shayba 21:488

Waki’ narrated to us, from Usama bin Zayd, from Sa’id al-Maqburi, from Abu Hurayrah, who said: A man intended to travel, so he came to the Prophet, peace and blessings of Allah be upon him, and said: "Advise me." He said: "I advise you to fear Allah, and to say the Takbir upon every elevated place."