Sahih Bukhari, Chapter: Prayers (Salat)

The Prophet saw expectoration (on the wall of the mosque) in the direction of the Qibla and scraped it off with his hand. It seemed that he disliked it and the sign of disgust was apparent from his face. He said, "If anyone of you stands for the prayer, he is speaking in private to his Lord, (or) his Lord is between him and his Qibla, therefore he should not spit towards his Qibla, but he could spit either on his left or under his foot." Then he took the corner of his sheet and spat in it, folded it and said, "Or do like this."