It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that when the Prophet (peace and blessings be upon him) set out on a journey and mounted his riding animal, he would say "Allahu Akbar" three times, then recite this supplication: "Glory be to the One who has subjected this to us, and we were not capable of subduing it, and indeed, to our Lord we will surely return." [Surah al-Zukhruf: 13] This hadith has a sahih (authentic) chain, but the two Shaykhs (Bukhari and Muslim) did not record it.