Yahya related to me from Malik from Nafi from Abdullah ibn Umarthat the Messenger of Allah, may Allah bless him and grant him peace,saw spittle on the wall of the qibla and scraped it off. Then he wentup to the people and said, "Do not spit in front of you when you arepraying, because Allah, the Blessed and Exalted, is in front of youwhen you pray."
USC-MSA web (English) reference: Book 14, Hadith 4
Arabic reference: Book 14, Hadith 461