It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) saw spittle, phlegm or nasal mucus on the qiblah wall, and he scraped it off.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3605
Hadith Gradingمحدثین:صحيح
Hadith Takhrij«صحيح۔ اخرجه البخاري 399۔ اخرجه مسلم 549»