Musnad Ahmad 27:1234

'Affan narrated to us, Hammad bin Salamah narrated to us, 'Ali bin al-Hakam informed us, from 'Ata' bin Abi Rabah, who said: A man was praising Ibn 'Umar. He said: So Ibn 'Umar began saying, "Like this," and he threw dust in his face. He said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "If you see the praisers, throw dust in their faces."