Musnad Ahmad 27:429

Yazid narrated to us, Muhammad informed us, from Nafi’, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) saw phlegm in the direction of the Qibla, so he took a stick or a pebble and scraped it off. Then he said: "When one of you stands to pray, let him not spit in the direction of his Qibla, for he is conversing with his Lord, Blessed and Exalted is He."