Musnad Ahmad 30:182

Sufyan narrated to us, Abu al-Zinad narrated to us, from al-A'raj, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you performs ablution, let him put water into his nose, then let him blow it out." And he said once: "Let him blow it out."