Musnad Ahmad 27:460

'Abd al-Razzaq narrated to us, Ibn Abi Rawwad informed us, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) prayed in the mosque and saw phlegm in the direction of the Qibla. When he finished his prayer, he said: "Indeed, when one of you prays in the mosque, he is conversing with his Lord, and Allah, Blessed and Exalted is He, faces him with His Face. So let none of you spit in the direction of the Qibla, nor to his right." Then he called for a stick and scraped it off, then he called for perfume (khaluq) and applied it.