آداب الأكل والشرب
Ibn ‘Umar (may Allah be pleased with him) reported: During the Prophet's lifetime, we would eat while walking and would drink while standing.
عن ابنِ عُمَرَ -رضي الله عنهما-، قال: كُنَّا على عَهْدِ رسولِ اللهِ -صلى الله عليه وسلم- نَأْكُلُ ونحنُ نَمْشِي، ونَشْرَبُ ونحنُ قِيَامٌ.
شرح الحديث :
The practice of the Companions (may Allah be pleased with him) proves that drinking while standing is permissible, because the Prophet (may Allah's peace and blessings be upon him) approved it. It is better, however, that man eats and drinks while s/he is seated, because this was the practice of the Prophet (may Allah's peace and blessings be upon him). He neither ate nor drank while standing. It was authentically reported that the Prophet (may Allah's peace and blessings be upon him) forbade drinking in a standing position. The Hadīth in question, however, indicates that drinking in a standing position is not absolutely prohibited; rather, doing so is not the proper attitude. In other words, it is better and more perfect that one drinks and eats in a seated position; however, it is permissible to do so while standing.