Musnad Ahmad 1101:16

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from al-Hakam, from Maymun ibn Abi Shabib, who said: A man began praising a governor of ‘Uthman. Al-Miqdad stood up and began throwing dust into his face. ‘Uthman said to him: "What is this?" He replied: "Indeed, the Messenger of Allah (peace and blessings of Allah be upon him) said: 'If you see the praisers, throw dust into their faces.'"