Musnad Ahmad 27:886

I read to Abd al-Rahman, [from] Malik, and Ishaq narrated to us, he said: Malik informed us, from Nafi’, that Abdullah ibn Umar said: The Messenger of Allah, may Allah bless him and grant him peace, saw spittle on the wall of the Qibla, so he scraped it off. Then he turned to the people and said: "When one of you is praying, let him not spit in front of his face, for Allah, the Mighty and Majestic, is in front of his face when he prays." Ishaq said in his narration: "spittle."