Musnad Ahmad 30:287

Isma'il narrated to us, Al-Qasim ibn Mihran narrated to me, from Abu Rafi', from Abu Hurayrah: That the Messenger of Allah (peace and blessings of Allah be upon him) saw phlegm in the direction of the Qiblah of the mosque. He turned to the people and said: "What is the matter with one of you who stands facing his Lord, then spits in front of him? Would one of you like to be faced and have someone spit in his face? If one of you spits, let him spit to his left, or under his foot. If he does not find [that possible], let him spit like this, in his garment." Al-Qasim described it: "He spat in his garment, then rubbed one part of it against the other."