Abu Sa'eed Al-Khudri RA narrates that Rasulullah (SAW) prohibited from blowing into any drink. (Riwayat Tirmidhi)
This Hadith has been graded as Hasan Sahih (sound and authentic) by Imam Tirmidhi RA.
It is quite unfortunate that due to the lack of religious education amongst the Muslim masses today, we find that this order from Rasulullah SAW is largely ignored. Drinking water should not be blown upon, even when it is for the intention of cooling hot/warm water down. Other than medicinal purposes which have been charted in Shari'ah, we should not make a habit of blowing into the water we consume. Some people unconsciously breath into the glass / cup when they drink. This is also wrong, and even recent modern medical studies have proven the detrimental acidic and septic effect on the water when this is done.