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. (DE)
English reference (DE): Book 31, Hadith 198 (DE)
Arabic reference (DE): Book 32, Hadith 207 (DE)