Is there a scriptural reference for this, please? I ask because it is likely that he told Sangha to take Refuge in the Dharma he revealed, and in each other as Sangha.
By Gautama's own admission, he was not the first Buddha as there had been others in the past.
Kāśyapa Buddha is said to be the one who proceeded Gautama in the Agammas and Nikayas. Here are a lot of the details about his background in the Nikayas.http://www.vipassana.info/ka/kassapa.htm
This is somewhat related to the thread on the Brahma Net Sutra actually -- that Shakyamuni has an original teacher who taught the Bodhisattva vows.
Let me quote Fazang's remarks on the relevant passage again:初法應爾故者。
1. The Dharma responds as such.
These three sets of pure precepts of the Bodhisattvas are the direct route to the site of enlightenment and the perfect cause for omniscience, therefore all the Buddhas manifesting in the world to benefit beings all rely on this ancient Dharma.
The Dharma being such: the first time one binds oneself to the Bodhisattva prātimokṣa is a requisite for the [ascertaining of the] origin of truth.
Like the Great King’s Way – the law being such, it is always regulated, so it must be explained.
《大方廣佛華嚴經疏》卷18〈15 十住品〉：「今初。謂三世佛果無不由此。十住因成。如大王路法爾常規。故同說也。」(CBETA, T35, no. 1735, p. 635, a9-10)
Thus it is stated below, “Each sit under the Bodhi tree and recite my original teacher’s precepts.”
《梵網經》卷2：「各坐菩提樹誦我本師戒」(CBETA, T24, no. 1484, p. 1004, a8-9)
Again it is said, “This Vairocana recites. I too recite it like this.”
《梵網經》卷2：「是盧舍那誦我亦如是誦」(CBETA, T24, no. 1484, p. 1004, a12)
When it says, “Buddha’s original teacher’s precepts,” it only states recite and does not say elucidate – it is clear that this original teaching is not a new regulation.
Again it is stated, “The Buddhas of the three realms have taught it, are teaching it and will teach it.” Therefore we know it is the same teaching.
So, it is clear from the Chinese Mahayana perspective Shakyamuni had a former teacher who presumably taught him the Bodhisattva vows which would include refuge vows as well.
Now, according to the Nikaya Pali tradition, of which I'm unfortunately not too well read, we find such remarks as this:
All-conquering, all-knowing am I, with regard to all things, unadhering. All-abandoning, released in the ending of craving: having fully known on my own, to whom should I point as my teacher? http://www.accesstoinsight.org/tipitaka ... ml#dhp-353
However, that doesn't preclude that in a past life Shakyamuni had taken refuge in a past Buddha. In fact Shakyamuni did quote the former Buddha (see the Amagandha sutta).
One other thing I would like to point out is that Maitreya was present in Shakyamuni's assembly and received the prophecy of his future Buddhahood. If I understand correctly, one receives a prophecy of future Buddhahood from a Buddha. In the case of Maitreya he was an ordained member of the sangha, so this would have included refuge vows. We can imagine Shakyamuni as well in a past life was in similar circumstances.
I don't know the Sanskrit title of the following Chinese Agama sutra (it translates as "Sutra of Past and Present Worlds and Times"), but in the passage quoted below Buddha speaks of Maitreya's future enlightenment and becoming a World Honoured Buddha.
覺。教化天上天下如今佛耶。」(CBETA, T01, no. 44, p. 830, b9-20)
Now as to a precise reference where Shakyamuni says, "I took refuge in such and such a past Buddha" I'll keep looking.
Maybe somebody can point to a precise place.
In any case I think I've provided enough sources so far to safely say we can infer that, indeed, Shakyamuni in a past life took refuge in a Buddha of the past.