Musnad Abu Ya'la al-Mawsili 6:120

Zuhayr narrated to us, Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Ishaq, 'Abdullah ibn Muhammad ibn Abi 'Atiq narrated to me, from 'Amir ibn Sa'd ibn Abi Waqqas, from his father, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "If one of you clears his throat while he is in the mosque, let him conceal his phlegm, so that it does not touch the skin of a believer or his garment and harm him."