Shama-il Tirmidhi Chapter 31, Hadith Number 2 (198).

Amr ibn Shu’aib radiyallahu anhu relates from his father, and he from his (Amr’s) grandfather, that he said, I had seen Rasoolullah sallallahu alaihe wasallam drinking water whilst standing and also whilst sitting.

There are many narrations mentioned wherein Sayyidina Rasoolullah sallallahu alaihe wasallam prohibited the drinking of water whilst standing. A narration of Sayyidina Abu Hurairah radiyallahu anhu is mentioned in Sahih Muslim that Sayyidina Rasoolullah sallallahu alaihe wasallam said, No person should stand and drink water, and if one forgetfully stood and drank it, it should be vomited out. Some ulama have summed both the deed and the prohibition in a few ways. Some ulama are of the opinion that the prohibition came later and repeals the previous law. Some ulama hold the opposite view that the narrations where he stood and drank water repeals the one that prohibits it. According to a well-known saying, the prohibition is not that of a Shar’ee or unlawful nature, but as an etiquette. It is also a blessing and kindness. Ibn Qayyim and others have mentioned the harm that standing and drinking causes. In short, Sayyidina Rasoolullah sallallahu alaihe wasallam standing and drinking shows it being permissible to do so, and that it is not haraam to stand and drink water. It is best not do so, as this is against the recommended practice and one will be committing a makrooh act by doing so.