Musannaf Ibn Abi Shayba 3:5073

Ibn ‘Ulayyah narrated to us, from al-Qasim ibn Mihran, from Abu Rafi‘, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) saw phlegm in the direction of the Qiblah of the mosque. He turned his face toward the people and said: "What is the matter with one of you who stands facing his Lord and then expectorates in His face? Would one of you like to be faced and then expectorated in his face? If one of you expectorates, let him expectorate to his left or under his feet." If he does not find [a place], let him spit like this into his garment. Then Isma‘il showed us that he spat into his garment and then rubbed it.