It was narrated that 'Abdullah bin Mas'ud said:
"The Messenger of Allah said: 'No one of you should bathe in open land or on a roof where he is not concealed; even if he does not see anyone, he can still be seen.'" (DE)
English reference (DE): Vol. 1, Book 1, Hadith 615 (DE)
Arabic reference (DE): Book 1, Hadith 658 (DE)