Nafi' said:
I accompanied Ibn ‘Umar who wanted to relieve himself. After doing so, in the course of his talk that day he said that a man going along a street met God’s messenger when he came out of the privy, or had passed water, and saluted him without receiving any response. Then when the man was almost going out of sight in the street God’s messenger struck his hands on a wall, wiped his face with them, struck once more and wiped his forearms, then responded to the man’s salutation, saying, “The only thing which prevented me from responding to your salutation was that I was not in a state of purity.”
Abu Dawud transmitted it.