The Buddhist monastic rule against killing a human being is obviously important, indeed fundamental. But the story of how the Buddha first came to pronounce it is inconsistent and implausible. On the one hand, it occurs in every version of the Buddhist legal code, the Vinaya, and therefore also in commentaries on those texts; on the other hand, it is hardly ever mentioned elsewhere. This article shows that the story came about through a misunderstanding of a phrase which we find in the Pali version of the rule. This misunderstanding is already present in the Pali canonical auto-commentary, so it is very ancient. Since they repeat virtually the same story, this also proves that parallel versions of the Vinaya preserved in Chinese may well depend on the Pali version (or something extremely close to it). On the other hand, evidence preserved in the Mahāsāṅghika Vinaya takes us back further towards the original, quite different, story of how the rule itself came into being. But even this presupposes the wording of the rule which is preserved, albeit with its meaning unrecognised, in the Pali Canon.