Bahz ibn Hakim reported from his father, from his grandfather that he Said, “O Messenger of Allah (SAW), from whom should we conceal that portion of our body that has to be concealed and from whom may we not conceal it?” He said, “Conceal it from everyone except your wife and female slave.” He submitted, “What if a man is with another man?” He said, as possible guard your satr that no one may see it.” He submitted, “What if a man is to himself?” He said, “Allah is more deserving that you show modesty to Him.”
[Abu Dawud 4017, Ahmed 20054]