Sahih Al Bukhari – Book of Ablutions (Wudu')
Volumn 001, Book 004, Hadith Number 144.
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Narated By Anas RA: Whenever the Prophet PBUH went to answer the call of nature, he (PBUH) 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)."
اللُّهُمَّ إِنِّي أَعُوذُ بِكَ مِنَ الْخُبْثِ وَالْخَبَائِثِ