"I depart with Allah's name, relying on Him. It is Allah who saves us from sins with His guidance (the ability to do so)." (Tirmidhi)
It is stated in the hadith that if a person recites the above dua and departs then there is an (unseen) procalamation that his need will be fullfilled and that he will be protected from any harm or injury. On hearing this dua, the Satan goes away from the place and refrains from troubling the reciting person.