The Buddha taught that he was the Bodhisattva Śvetaketu who resided in the Tushita heavans, before being born as the human Prince Gautama.
where is this passage coming from?
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Śvetaketu and Bodhisattva in Sanskrit are Setaketu and Bodhisatta in Pali.
There was one person called Setaketu in the Setaketu Jataka, but this is the story of a human on Earth, but I'm not sure if this is the same as Śvetaketu of the Chandogya Upanishad. In any case, this Setaketu is not a Bodhisatta.
From this page, we find this story:
And, having ended his life span in the human abode, the future Buddha was reborn as a deva by the name of Setaketu in the abode of Tusitā. ....
In this way, after making the five great investigations, the Bodhisatta Deva Setaketu resolved: “I will descend to the human abode and become a Buddha.” Having so resolved, to the devas and Brahmās from the ten thousand world-systems who had assembled to request him, the Bodhisatta gave his consent thus: “O devas and Brahmās, now is the time for me to become a Buddha as requested by you. You may now take leave as you please, I will go down to the human abode to attain Buddhahood.”
That story of Bodhisatta Setaketu appears to be from the Buddhavamsa.
From the wikipedia page on Buddhavamsa:
The Buddhavaṃsa (also known as the Chronicle of Buddhas) is a hagiographical Buddhist text which describes the life of Gautama Buddha and of the twenty-four Buddhas who preceded him and prophesied his attainment of Buddhahood. It is the fourteenth book of the Khuddaka Nikāya, which in turn is the fifth and last division of the Sutta Piṭaka. The Sutta Piṭaka is one of three pitakas (main sections) which together constitute the Tripiṭaka, or Pāli Canon of Theravāda Buddhism.
Along with the Apadāna and the Cariyāpiṭaka, the Buddhavaṃsa is considered by most scholars to have been written during the 1st and 2nd century BCE, and is therefore a late addition to the Pāli Canon.
Apparently the Buddha spoke to Sariputta and the contents of this is recorded in the Buddhavamsa.
The Buddhavamsa, Cariyāpiṭaka, Apadāna and Jataka Tales are later additions to the Pali Canon, a few centuries after the Buddha's passing. I believe it was not recited by Ananda at the First Buddhist Council right after the Buddha's passing.
Furthermore, I could not find the Buddha ever mentioning Bodhisatta Setaketu in the main nikayas of the Sutta Pitaka (by searching AccessToInsight and SuttaCentral).
So, it is hard to concretely state that it was the historical Buddha who first spoke about Setaketu.
As such, in my opinion, this can be considered an unverifiable, though inspiring, legend.
After writing this answer, I discovered that the Acchariyaabbhuta Sutta (MN 123) has Ven. Ananda talking about the Bodhisatta before his birth, in the presence of the Buddha, apparently recalling something the Buddha told him before, but he does not explicitly mention the name "Setaketu". Tusita heaven (host of Joyful Gods) is mentioned.
From Acchariyaabbhuta Sutta (MN 123):
“Sir, I have heard and learned this in the presence of the Buddha: ‘Mindful and aware, the being intent on awakening was reborn in the host of Joyful Gods.’ This I remember as an incredible quality of the Buddha.
I have learned this in the presence of the Buddha: ‘Mindful and aware, the being intent on awakening remained in the host of Joyful Gods.’ This too I remember as an incredible quality of the Buddha.
I have learned this in the presence of the Buddha: ‘For the whole of that life, the being intent on awakening remained in the host of Joyful Gods.’ This too I remember as an incredible quality of the Buddha.
I have learned this in the presence of the Buddha: ‘Mindful and aware, the being intent on awakening passed away from the host of Joyful Gods and was conceived in his mother’s womb.’ This too I remember as an incredible quality of the Buddha.
If you're interested in topics surrounding Bodhisattva-hood, you can read the book "The Genesis of the Bodhisattva Ideal" by Ven. Anālayo.