Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him) says: I heard the Prophet (peace and blessings be upon him) say: "When any of you spits in the mosque, he should conceal his spittle (in the soil) so that it does not touch the skin or the clothing of a believer and harm him."
Ahmad and Abu Ya'la recorded it; its narrators have been deemed trustworthy (muwaththaq).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2001