Musannaf Ibn Abi Shayba 3:5065

Abu Usama narrated to us, from al-Mujalid, from al-Sha’bi, that the Prophet (peace and blessings of Allah be upon him) saw phlegm in the direction of the qibla of the mosque. He stood up to it and scraped it off with his hand, then he called for perfume (khaluq). ‘Amir said: "It is a Sunnah."