Musnad Ahmad 1236:2

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from Mansur, he said: I heard Rib'i ibn Hirash, from Tariq ibn Abdullah, from the Prophet, may Allah bless him and grant him peace, that he said: "When you pray, do not spit in front of you, nor to your right, but spit toward your left if it is empty. Otherwise, place your foot [on it] and rub it."