رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 768
The Book About the Etiquette of Eating
Permission to Drink While Standing
وَعَنِ ابْنِ عُمَرَ رضى اللَّهُ عَنْهُمَا قَالَ:كُنَّا نَأْكُلُ عَلَى عَهْدِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَنَحْنُ نمشى ، وَنَشْرَبُ وَنَحْنُ قِيَامٌ.((رَوَاهُ التِّرْمِذِيُّ))
Ibn 'Umar (May Allah be pleased with them) reported: During the lifetime of the Messenger of Allah (ﷺ) we would eat while walking and would drink while standing. [At- Tirmidhi].
Riyad as-Salihin, 768