In the BU 1.4.10 we find this passage (Olivelle's translation)
In the beginning this world was only brahman, and it knew only itself (ātman), thinking: "I am brahman." As a result it became the whole. Among the gods, likewise, whosoever realized this, only they became the Whole. It was the same also among the seers and among the humans. Upon seeing this very point, the seer Vāmadeva proclaimed: "I was Manu, and I was the sun." This is true even now. If a man knows "I am Brahman" in this way, he becomes this whole world.Anyone interested in the Sabba Sutta should pay close attention to this verse as this is also the context for that sutta - Olivelle's 'Whole' is a translation of Sanskrit sarvaṃ = Pāli sabbaṃ - but for this essay I want to draw attention to the phrase "I am brahman": ahaṃ brahmāsmi. This is seen by Vedic believers as a kind of credo. It sums up the path according to the sages of the Upaniṣads which is that the realisation that you are brahman is the highest realisation. In this realisation one becomes this whole world (sa idaṃ sarvaṃ bhavati).
In the Pāli Kevaddha Sutta the householder Kevaddha approaches the Buddha to encourage him to perform some miracles and thereby attract followers. The Buddha says that not how he operates. How he does operate is spelled out in the long passage that is repeated in all 13 of the first of the Dīgha Nikāya suttas, but this segues into a story of a monk who, desiring to know where the elements cease without remainder. In order to answer the question he attains super human states of consciousness in meditation and visits the realms of the various devas, moving up the scale until me meets Brahmā himself. Posed the question Brahma can only reply:
"ahamasmi, bhikkhu, brahmā mahābrahmā abhibhū anabhibhūto aññadatthudaso vasavattī issaro kattā nimmātā seṭṭho sajitā vasī pitā bhūtabhabyānan" ti.Note that Brahmā doesn't answer the question. It turns out that he doesn't know the answer, but has to keep up appearances because the other gods believe it is true that Brahmā is the omnipotent creator. He takes the monk to one side to explain this and point him back in the direction of the Buddha.
"I am, bhikkhu, Brahmā, Great Brahmā, unconquered conqueror, omnipotent, Lord over all, maker and creator, the highest, controller of the cosmic order, and father of all beings past, present and future."
But notice how he starts his answer. If we leave out the 'bhikkhu' he says: ahamasmi brahmā. Compare this to the Sanskrit: ahaṃ brahmāsmi. That the Pāli is a reference to the BU, or at least to the body of teaching recorded in that text, is clear. Although the BU was not written down for many years after the Buddhist texts, the scholarly consensus is that BU represents a body of teachings that predate the Buddha by several centuries. Given the flexibility of syntax in the two languages we are looking at the same statement. Exactly the same except that the Sanskrit has an ambiguity - brahmāsmi can be read as brahma asmi or brahmā asmi i.e. as the neuter or masculine. The first is the abstract universal essence of the cosmos that manifests as ātman in the individual; the second is the masculine creator god. The first usage in BĀU 1.4.10 is the context of a neuter pronoun 'it' (tad), while the second is in the context of a masculine pronoun 'him' (sa), so both senses could be being used here! Gombrich observes that the Buddha has selected the less abstract, and therefore less sophisticated, of the two, i.e. Brahmā as creator god, and that this helps to contribute to the overall sense of this being not just a polemic, but a parody. Johannes Bronkhorst has been very critical of Gombrich's interpretation of this kind of reference as evidence of the Buddha's sense of humour,  but personally I think this example is funny. On the one hand the realisation "I am Brahmā" encapsulates the highest goals of religion; and on the other the statement is just an egotistical and deluded claim with no basis.
The ideal of union with Brahmā (brahmasahavyatā) is also found in the Tevijja Sutta (DN 13) where we find the Buddha informing some hapless Brahmins Vāseṭṭha and Bhāradvaja  that though there own teachers are ignorant of the way leading to this goal, that:
brahmānaṃ cāhaṃ, vāseṭṭha, pajānāmi brahmalokañca brahmalokagāminiñca paṭipadaṃThe Buddha then teaches the meditations we have come to know as the brahmavihāra 'dwelling with Brahmā', though the name is not used here. Brahmavihāra is actually a synonym of brahmasahavyatā. It would be like walking into a Christian church and asking "How many of your priests have been face to face with God? None? I have, and I can tell you how to be in His presence. You don't have to die and go to heaven, you can dwell in heaven right now!" - and teaching the mettābhāvanā! I've often wondered what would happen if we took the Buddha's approach to theistic religion. Forget about opposition and proving that God exists, but just roll with it and teach Buddhism in Christian terms. I think most of us are too afraid of losing our religion, and perhaps lack confidence in our methods, to even try this. And, of course, it would require one to be truthfully in that state of dwelling with God (brahmavihāra). But it is what the Buddha appears to have done.
I know Brahmā, Vāseṭṭha, and Brahmā's domain, and the way leading to Brahmā's domain.
To those people who claim that Buddhism is a religion which tolerates all views this must come as a shock. Not only did the Buddha not tolerate wrong views, he actively went about subverting them and making fun of people who held them. There are times when the Buddha of the Pāli Canon makes Richard Dawkins seem like an appeaser.
- Professor Gombrich's contribution is summed up in his book What the Buddha Thought. References to his individual papers can be found there. The observations I make here has been observed by him previously, but I'm putting them in my own words.
- also Kevaṭṭa Sutta. Dīgha Nikāya 11. PTS D i.211. Translation that follows is mine. Pāli text from CST.
- Especially in his book Greater Magadha: Studies in the Culture of Early India.
- These two show up in various retellings of this story at e.g. DN 13, MN 98, and Sn 3.9. I haven't yet done a detailed comparison, but I'm working on it.