Musannaf Ibn Abi Shayba 3:4172

Abu al-Ahwas narrated to us, from Abu Hamza, from Abu Salih, that a relative of Umm Salama prayed and blew [air from his mouth]. Umm Salama said: "Do not do that, for the Messenger of Allah (peace and blessings of Allah be upon him) said to a black servant of ours who was called Rabah: 'Make your face dusty, O Rabah.'"