Sahih Muslim Book 003, Hadith Number 0669.
Chapter : It is permissible to take: a bath naked in complete privacy.
Amongst the traditions narrated from Muhammad, the Messenger of Allah (may peace be upon him) on the authority of Abu Huraira, the one is that Banu Isra'il used to take a bath naked, and they looked at the private parts of one another. Moses (peace be upon him), however, took a bath alone (in privacy); and they said (tauntingly): By Allah, nothing prohibits Moses to take a bath along with us, but sacrotal hernia. He (Moses) once went for a bath and placed his clothes on a stone and the stone moved on with his clothes. Moses ran after it saying: O stone, my clothes, O stone, my clothes, and Banu Isra'il had the chance to see the private parts of Moses, and said: By Allah, Moses does not suffer from any ailment. The stone then stopped, till Moses had been seen by them, and he then took hold of his clothes and struck the stone. Abu Huraira said: By Allah, there are the marks of six or seven strokes made by Moses on the stone.