Maliks Muwatta Book 14, Hadith Number 5.

Section : The Prohibition against Spitting towards the Qibla.

Yahya related to me from Malik from Hisham ibn Urwa from his father from A’isha, the wife of the Prophet, may Allah bless him and grant him peace, that the Prophet, may Allah bless him and grant him peace, saw spittle, or mucus or phlegm, on the wall of the qibla and scraped it off.