Musnad Ahmad 1101:20

Waki’ and ‘Abd al-Rahman narrated to us, they said: Sufyan narrated to us, from Mansur, from Ibrahim, from Hammam ibn al-Harith, who said: A man came to ‘Uthman and praised him to his face. He said: So al-Miqdad ibn al-Aswad began to throw dust into his face and said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded us that when we meet the praisers, we should throw dust into their faces."