It was narrated from Jabir bin `Abdullah that:
"A man passed by the Prophet while he was urinating, and greeted him by the Salam. The Messenger of Allah said to him: "If you see me in this situation, do not greet me with the Salam, for if you do that I will not respond to you.'" (JA)
English reference (JA): Vol. 1, Book 1, Hadith 352 (JA)
Arabic reference (JA): Book 1, Hadith 380 (JA)