Bukhari (142), Muslim (375), Tirmidhi (7)

Bukhari 142, Muslim 375

Narrated by Anad: Whenever the Prophet (pbuh) 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)."
(اللهم اني اعوذ بك من الخبث والخبائث)

Sahih Bukhari, Book of Ablutions, Hadith No. 142