Anas(Radi-Allahu 'anhu):

 

reported that Allah's Apostle (may peace be upon him) forbade that a person should drink while standing. Qatada reported: We said to him: What about eating? Thereupon he (Anas)

 

(Radi-Allahu 'anhu):

 

said: That is even worse and more detestable (abominable).

Source: Sahih Muslim
Download: https://market.android.com/details?id=com.triosLabs.SahihMuslim

Medical Commentary:

Stomach damage : when we stand and drink water, it flows easily and in great volume down the food canal and splashes onto the lower stomach wall. This splash damages the stomach wall and the nearby organs. Long term practice disturbs the digestive system and further causes heart  and kidney problems.

Kidney damage : water intake while standing lets the water pass through kidneys with a push without much filtration. This may cause impurities to gather in the bladder  or in blood. Now kidneys ,bladder and heart are at stake.

Arthritis : if water is taken while standing it disturbs the balance of other body fluids. Ultimately it also affects the joint regions and cause deficiency of required fluids in joints. This results in joint pains known as arthritis.

http://easeyourlyf.blogspot.in/2012/09/do-not-drink-water-while-standing.html#.UeeMPkEwfX5

also 

Visit this link to see some healthy discussion by various religions' followers who recommend the same thing (Drinking water while sitting)

http://www.empowher.com/community/ask/better-posture-drink-water