Abdul Hameed bin Bhram عبد الحميد بن بهرام الفزاري المدائني · The Book of Purification and its Sunnah

The Messenger of Allah said: "No two people should converse while relieving themselves, each of them looking at the private parts of the other, for Allah, the Mighty and Sublime, hates that." (Da'if) Other chains with similar wording.