Musnad Ahmad 1101:21

'Abd al-Rahman narrated to us, from Sufyan, from Habib, from Mujahid, from Abu Ma'mar, who said: A man stood up praising one of the commanders, so al-Miqdad began throwing dust into his face and said: "The Messenger of Allah, peace and blessings of Allah be upon him, commanded us to throw dust into the faces of the praisers."