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