Sahih Bukhari Volume 001, Book 004, Hadith Number 144.

Narated By Anas : Whenever the Prophet went to answer the call of nature, he used to say, "Allah-umma inni a'udhu bika minal khubuthi wal khaba'ith i.e. O Allah, I seek Refuge with You from all offensive and wicked things (evil deeds and evil spirits)."