Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain of narration (from Sayyiduna Ibn Umar, may Allah be pleased with them both), it is narrated that the Messenger of Allah, peace and blessings be upon him, saw spit on the wall facing the qiblah, so he scraped it off (and cleaned it), then turned towards the people and said: "When any one of you prays, he should not spit in front of himself, because when he prays, Allah is in front of him."