Musnad Ahmad 8:104

Ibn Abi ‘Adi narrated to us, from Ibn Ishaq, and Ya‘qub narrated to us, from my father, from Muhammad ibn Ishaq, he said: ‘Abdullah ibn Muhammad—Ya‘qub ibn Abi ‘Atiq said—narrated to me, from ‘Amir ibn Sa‘d, who narrated to him from his father, Sa‘d, he said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "If one of you clears his throat (spits) in the mosque, let him conceal his phlegm, lest it strike the skin of a believer or his garment and harm him."