Abu Sa’eed al-Khudri (may Allaah be pleased with him) narrated that the Prophet (peace and blessings of Allaah be upon him) forbade blowing into one’s drink. [Narrated by al-Tirmidhi 188]
Tirmidhi said: a saheeh hasan hadeeth. It was also classed as saheeh by Ibn al-Qayyim in I’laam al-Muwaqqi’een (4/317).
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in Sharh Riyadh al-Saaliheen (2/457):
“That is because if a person blows (into the vessel), some harmful things may come with the air that is exhaled, such as disease and the like. But some of the scholars made an exception in cases of necessity, such as if the drink is hot and he needs to drink it quickly. In that case some scholars granted a concession. But it is better not to blow on it, even if it is hot. If it is hot and he has another vessel, then he may pour some of it into that vessel then pour it back until it cools down. ”
Once the reason for the prohibition on blowing or breathing into the vessel was understood, the scholars drew an analogy with everything that may lead to contamination of food and drink.
Al-Shawkaani said in Nayl al-Awtaar (8/221):
Just as one should not breathe into the vessel, one should not burp into it either.
He also says in Nayl al-Awtaar:
“...as for his saying ((or blowing in it)) ie: in the vessel which is drunk from. The word ((vessel)) here includes the vessel which is used for food and drink. So one should not blow in the vessel in order to remove that which is in the water from dirt or what is similar to that, for verily blowing generally is not free from saliva which is deemed unclean or filthy. Likewise one should not blow in the vessel in order to cool down hot food, rather he should be patient until the food has cooled down itself as previously mentioned. One should also not eat the food whilst it is hot, for verily the blessing will depart from it and it is from the food of the people of the hellfire.”