In one sutta a monk was teaching, that there's something that is behind our existence, that is permanent (opposite of non-self). Buddha asked for this explicitely, got confirmed, and corrected the view. Which sutta it is?
Possibly MN 38, where the monk held:
At one time the Lord was staying near Sāvatthī in the Jeta Grove in Anāthapiṇḍika's monastery. Now at that time a pernicious view like this had accrued to the monk called Sāti, a fisherman's son: “In so far as I understand Dhamma taught by the Lord it is that this [same; identical; anañña] consciousness itself runs on, fares on, not another."
Note: I chose the translation of I.B. Horner to avoid the contradictory eternalism of the other translators, who use words such as "transmigration" or add words that do not even exist in the sutta, such as "round of rebirths".
Possibly MN 35, however the protagonist is not a bhikkhu but is a Jain:
All the plants and seeds that achieve growth, increase and maturity do so depending on the earth and grounded on the earth. All the hard work that gets done depends on the earth and is grounded on the earth.
In the same way, an individual’s self (attāyaṃ purisapuggalo) is form. Grounded on form they make good and bad choices. An individual’s self is feeling … perception … choices … consciousness. Grounded on consciousness they make good and bad choices.
Aggivessana, are you not saying this: ‘Form is my self, feeling is my self, perception is my self, choices are my self, consciousness is my self’?
Indeed, Master Gotama, that is what I am saying.