It is fine to either sit or stand when drinking water, because the Prophet Muhammad did both.
What the Qur’an and Sunnah Say
Abdullah ibn Abbas says, “I served Zamzam water to the Messenger of Allah and he drank from it while standing.” (Bukhari #1528, Muslim #2027)
Anas ibn Malik says that the Messenger of Allah disapproved of drinking water while standing. (Muslim #2024)
Abu Hurayrah states that the Messenger of Allah said, “None of you should drink while standing. If you forget you should vomit.” (Muslim #2026) Note that no one understood this literally, or even considered it prohibited to stand and drink.
What the Scholars Said
Scholars of the Maliki and Hanbali school said that it is fine to either stand or sit and drink, since the Prophet’s action takes precedence over his words. (Mardawi, Al-Insaf, 8:330; Ibn Juzayy, Al-Qawanin Al-Fiqhiyya, 646)
Scholars of the Hanafi school said that it is recommended to stand and drink Zamzam water but disliked to stand when drinking anything else. (Radd al-Muhtar 2:524)
Scholars of the Shafii school state that it is disliked to stand when drinking anything, even Zamzam water. (An-Nawawi, Al-Minhaj Sharh Sahih Muslim, 1532)
Shaykh Mustafa Umar