Musnad Ahmad Hadith 7533

Once, Muslim (may Allah have mercy on him) asked Abu Hurairah (may Allah be pleased with him) about the issue of drinking water while standing. He replied, "Nephew! I have seen that once the Noble Prophet (ﷺ) tied his mount, which was sitting, and I was holding its reins in such a way that my foot was on its leg. Meanwhile, some people from the Quraysh came and stood around the Noble Prophet (ﷺ). During this time, a vessel of milk was brought to the Noble Prophet (ﷺ). The Noble Prophet (ﷺ) drank it while sitting on the mount, then graciously passed it to the person on his right. He drank it while standing, and thus all the people drank that milk while standing."