Shama-il Tirmidhi Chapter 31, Hadith Number 1 (197).

Ibn Abbas radiyallahu anhu says, Rasoolullah sallallahu alaihe wasallam drank Zam-zam water while he was standing.

Sayyidina Rasoolullah sallallahu alaihe wasallam has prohibited the drinking of water whilst standing. This shall be discussed in the commentary of the next hadith. For this reason some ulama have prohibited the drinking of Zam-zam also whilst standing. They say that he drank it whilst standing because of the huge crowds and the rush, or as a concession. The well-known view of the ulama is that Zam-zam is not included in this prohibition. It is better to stand and drink it.