Musannaf Ibn Abi Shayba 28:1095

Waki’ narrated to us, from Usama bin Zayd, from Sa’id al-Maqburi, from Abu Hurayrah, who said: A man intended to go on a journey, so he came to the Prophet, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, advise me." He said: "I advise you to fear Allah (taqwa) and to say the takbir (Allahu Akbar) upon every elevated place."