Musnad Ahmad 30:2243

'Affan narrated to us, he said: Shu'bah narrated to us, he said: Qasim ibn Mihran informed me, he said: I heard Abu Rafi' narrating from Abu Hurairah, that the Prophet (peace and blessings of Allah be upon him) saw phlegm in the direction of the Qiblah. He said: He would say another time, "He scraped it off." He said: Then he said, "I stood up and scraped it off." Then he said, "Would any of you like, if he were in his prayer, for someone to spit in his face, or blow in his face? If one of you is in his prayer, let him not spit in front of him, nor to his right, but to his left, under his foot. If he does not find [a place], he said with his garment like this."