'Aishah (May Allah be pleased with her) said:
The Messenger of Allah (ﷺ) saw spittle or snot or sputum, sticking to the wall towards Qiblah and scratched it off.[Al-Bukhari and Muslim]. (ID)
App reference (ID): Book 18, Hadith 184 (ID)
Arabic/English book reference (ID): Book 18, Hadith 1694 (ID)