Reference : Hisn al-Muslim 10

(بِسْمِ الله)
اللّهُـمَّ إِنِّـي أَعـوذُ بِـكَ مِـنَ الْخُـبْثِ وَالْخَبائِث
[Bismillāhi]
 Allāhumma 'innī 'a`ūdhu bika minal-khubthi walkhabā'ith.

[In the Name of Allah]. 
O Allah, I seek protection in You from evil and the evil ones 
(or, from the evil male and female Jinn).

Reference:
Al-Bukhari 1/45, Muslim 1/283. The addition of Bismillah at its beginning 
was reported by Said bin Mansur. See Fathul-Bari 1/244

Chapter: Before entering the bathroom