Musnad Abu Ya'la al-Mawsili 160:40

Zakariya ibn Yahya narrated to us, Hushaym narrated to us, from al-Qasim ibn Mihran, Abu Rafi’ narrated to us, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you is in his prayer, let him not spit toward the qiblah, nor to his right, but rather under his left foot. If he is unable to do so, let him spit into the side of his garment, then fold one part of it over the other." Abu Hurayrah said: "It is as if I am looking at the Messenger of Allah (may Allah bless him and grant him peace) folding his garment, one part over the other."