كتـــــاب أدب الطعام
3
The Book About the Etiquette of Eating
'Amr bin Shu'aib on the authority of his father and grandfather reported:
that they saw Messenger of Allah (ﷺ) drink standing, and sitting.

[At-Tirmidhi].

The last three Ahadith point out the permissibility of eating and drinking while walking or standing or sitting, but drinking in the sitting position remains the best.
وعن عمرو بن شعيب عن أبيه عن جده رضى الله عنه قال‏:‏ رأيت رسول الله صلى الله عليه وسلم يشرب قائماً أو قاعداً‏.‏ ‏‏.‏
Sunnah.com reference : Book 3, Hadith 43
Arabic/English book reference : Book 3, Hadith 770