In a Hadith recorded by Imam Bukhari in his Sahih (Book of Hajj, Chapter, what has been mentioned about Zamzam, Hadith no. 1529), it is mentioned that the Prophet (PBUH) was offered Zam Zam water, and he drank it standing up. In accordance with this practice, we too stand when we drink Zam Zam water. Ordinary water and drinks should be consumed whilst sitting. Anas (may Allah be pleased with him) reports that the Prophet (peace and blessings of Allah be upon him) forbade from drinking whilst standing (Sahih Muslim, Book of Drinks, Chapter; the disliked nature of drinking whilst standing, Hadith no. 3772).