"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.'" (Using translation from Ibn Mājah 352)
"إذا رأيتنى على مثل هذه الحالة (يعني البول) فلا تسلم عليَّ فإنك إن فعلت ذلك لم أرد عليك".