Fortress of the Muslim (Hisn al-Muslim)
حصن المسلم
(5) Chapter: Before undressing
(5) ما يقول إذا وضع الثوب
Hisn al-Muslim 9 
Bismillāhi In the Name of Allah. Reference: At-Tirmidhi 2/505, among others. See 'Irwa'ul Ghalil no. 49 and Sahihul-Jami' 3/203
بِسْمِ الله
Reference : Hisn al-Muslim 9