Musannaf Ibn Abi Shaybah

Hazrat Ata bin Abu Rabah narrates that a man was mentioning the praise of someone in the presence of Hazrat Ibn Umar (may Allah be pleased with them). So Hazrat Ibn Umar (may Allah be pleased with them) began to throw dust on his face with his fingers, and said that the Messenger of Allah (peace and blessings be upon him) said: "When you see those who praise others, throw dust into their mouths."