None of you should drink while standing, and whoever forgets, then let him induce vomit

Anas (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) forbade drinking while standing. Then Qatādah ibn Di‘āmah As-Sadūsi (may Allah have mercy upon him) said: "We asked Anas (may Allah be pleased with him) about the ruling on eating while standing? Is it forbidden like drinking? Anas replied: 'That is even worse – or more detestable (than drinking while standing).'"

The Prophet (may Allah's peace and blessings be upon him) forbade that anyone drinks while he is standing, and if someone forgot and drank while standing, he should empty his stomach. This is only recommended (not obligatory), so he will not bear sin if he does not vomit what he drank, because drinking while standing is disliked, not prohibited.