According to the texts (Cv 10), after the Buddha returned to visit his father in Kapilavatthu, his aunt and step-mother, Mahapajapati Gotami, followed him back to Vesali and eventually ordained her using the eight garu dhammas as her higher ordination. He then gave the injunction that the male monks were to ordain the female monks, which was the beginning of the bhikkhuni sangha.
1. Now Mahā-pajāpatī the Gotamī went up to the place where the Blessed One was, and bowed down before him, and stood respectfully on one side. And, so standing, Mahā-pajāpatī the Gotamī spake thus to the Blessed One: 'What course, Lord, should I pursue towards these women of the Sākya clan?'
Then the Blessed One taught Mahā-pajāpatī the Gotamī and incited her, and aroused her, and gladdened her with religious discourse; and she, so taught, incited, aroused, and gladdened, bowed down before the Blessed One, and keeping him on net right hand as she passed him, she departed thence.
Then the Blessed One, in that connexion, delivered a religious discourse, and said to the Bhikkhus, 'I allow Bhikkhunīs, O Bhikkhus, to receive the upasampadā initiation from Bhikkhus.'
2. Now those Bhikkhunīs said to Mahā-pajāpatī the Gotamī: 'Neither have you received the upasampadā initiation, nor have we; for it has thus been laid down by the Blessed One: "Bhikkhunīs are to be initiated by Bhikkhus."'
Then Mahā-pajāpatī the Gotamī went to the venerable Ānanda, and [repeated their words to him]. And the venerable Ānanda went to the Blessed One, and [repeated them to him].
'In that moment, Ānanda, when Mahā-pajāpatī the Gotamī took upon herself the Eight Chief Rules, that was to her as the upasampadā initiation.'
-- Cv 10.2 (Rhys-Davids, trans)