Musnad Ahmad 27:1295

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, Layth ibn Abi Sulaym narrated to us, from Nafi', from Ibn 'Umar, from the Prophet, peace and blessings of Allah be upon him, who said: "When one of you prays, let him not spit in the direction of the Qiblah, for the Most Merciful is in front of him, nor to his right, but to his left, or under his left foot."