Musnad Ahmad 32:2004

Yazid ibn Harun narrated to us, Shu'bah informed us, from Qatadah, from Anas, from the Prophet, may Allah bless him and grant him peace, who said: My father said: And al-Dahhak, meaning Ibn Makhlad, narrated to me, he said: Shu'bah informed us, from Qatadah, from Anas, from the Prophet, may Allah bless him and grant him peace. And my father narrated to me, Muhammad ibn Yazid al-Wasiti narrated to us, from Hisham al-Dastawa'i and Shu'bah, both of them from Qatadah, from Anas ibn Malik, from the Prophet, may Allah bless him and grant him peace, who said: "Spitting"—and Yazid and al-Dahhak ibn Makhlad said in their narration: "The phlegm"—in the mosque is a sin, and its expiation is burying it."