"Mystics" in buddhism? ("There is nothing hidden in my teaching" and the like)
It's the Mahaparinibbana Sutta, DN 16:

31. And the Blessed One recovered from that illness; and soon after his recovery he came out from his dwelling place and sat down in the shade of the building, on a seat prepared for him. Then the Venerable Ananda approached the Blessed One, respectfully greeted him, and sitting down at one side, he spoke to the Blessed One, saying: &quot;Fortunate it is for me, O Lord, to see the Blessed One at ease again! Fortunate it is for me, O Lord, to see the Blessed One recovered! For truly, Lord, when I saw the Blessed One's sickness it was as though my own body became weak as a creeper, every thing around became dim to me, and my senses failed me. Yet, Lord, I still had some little comfort in the thought that the Blessed One would not come to his final passing away until he had given some last instructions respecting the community of bhikkhus.&quot;
32. Thus spoke the Venerable Ananda, but the Blessed One answered him, saying: &quot;What more does the community of bhikkhus expect from me, Ananda? I have set forth the Dhamma without making any distinction of esoteric and exoteric doctrine; there is nothing, Ananda, with regard to the teachings that the Tathagata holds to the last with the closed fist of a teacher who keeps some things back. Whosoever may think that it is he who should lead the community of bhikkhus, or that the community depends upon him, it is such a one that would have to give last instructions respecting them. But, Ananda, the Tathagata has no such idea as that it is he who should lead the community of bhikkhus, or that the community depends upon him. So what instructions should he have to give respecting the community of bhikkhus?
(source)

The Pali is simply:

natthānanda tathāgatassa dhammesu ācariyamuṭṭhi.
Ananda, there is no teacher's fist in regards to the dhammas of the Tathāgata.