[42] Now on that occasion several bhikkhunīs (bhikkhunī), travelling to Sāvatthī in the Kosalan country, arrived at a certain village in the evening. Among them a certain bhikkhunī was beautiful, lovely to behold, graceful. A certain man, on seeing that bhikkhunī, became bound to her in mind. Then that man, while preparing sleeping-places for those bhikkhunīs, prepared the sleeping-place for that bhikkhunī off to one side. Then that bhikkhunī, having observed, "This man is obsessed; if he comes during the night, there will be trouble for me," went, without informing the bhikkhunīs, to a certain family and made her sleeping-place there. Then that man, coming in the night and searching for that bhikkhunī, jostled the bhikkhunīs. The bhikkhunīs, not seeing that bhikkhunī, said thus: "Without doubt that bhikkhunī has gone out with a man."
Then, when that night had passed, that bhikkhunī approached those bhikkhunīs. The bhikkhunīs said this to that bhikkhunī: "Why, lady, did you go out with a man?"
"I did not go out with a man, ladies," she said, and reported the matter to the bhikkhunīs.
Those bhikkhunīs who were of few wishes … they complained, criticized, and spread it about: "How indeed can a bhikkhunī spend the night alone, apart?" …
"Is it true, as they say, bhikkhus, that a bhikkhunī spends the night alone, apart?"
"It is true, Blessed One."
The Buddha, the Blessed One, rebuked them: "How indeed, bhikkhus, can a bhikkhunī spend the night alone, apart? This, bhikkhus, is not for the confidence of those without confidence, nor for the increase of those who have confidence …
"And thus, bhikkhus, let the bhikkhunīs recite this training rule (sikkhāpada): Should any bhikkhunī go alone among the villages, or go alone to the far bank of a river, or spend the night alone, apart, this bhikkhunī too has fallen into an act incurred at the first transgression (paṭhamāpattika), a saṅghādisesa entailing being sent away (nissāraṇīya)."
And thus this training rule was laid down by the Blessed One for the bhikkhunīs.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.