Musnad Ahmad 31:215

Yahya ibn Sa'id narrated to us, from Malik, Ayyub ibn Habib narrated to me, from Abu al-Muthanna, who said: I was with Marwan when Abu Sa'id entered and said: "Did I hear the Messenger of Allah, peace and blessings of Allah be upon him, forbid blowing into a drink?" He said: "Yes." A man said: "I do not quench my thirst in one breath." He said: "Move it away from you, then breathe." He said: "I see a speck of dirt in it." He said: "Then pour it out."