Musnad Ahmad 31:289

Waki’ narrated to us, from Malik ibn Anas, from Ayyub ibn Habib, the freed slave of Banu Zuhrah, from Abu al-Muthanna al-Juhani, who said: I was sitting with Marwan ibn al-Hakam, and Abu Sa’id al-Khudri entered. Marwan said to him: "Did you hear the Prophet, may Allah bless him and grant him peace, forbidding blowing into a drink?" He said: "Yes." A man said to him: "But I do not quench my thirst in one breath." He said: "Remove it from your mouth, then breathe." He said: "What if I see a speck of dirt?" He said: "Then pour it out."