Musannaf Ibn Abi Shayba 34:233

Abu Mu'awiyah narrated to us, from Asim, from Abbas ibn Abdullah al-Hashimi, who said: The first [instance of] the mosques being perfumed was that the Messenger of Allah (may Allah bless him and grant him peace) saw phlegm in the direction of the Qibla, so he scraped it off. Then he ordered perfume (khuluq), and it was smeared over its place. Thereafter, the people began to perfume the mosques.