His students asked him what he saw. He said, "The scroll is burning but the letters are flying [up to heaven.]" They told him to open his mouth and let the fire in, that he should die more quickly and suffer less. He said, "It is better that he who gave [me life] should take it [from me.]" There was a Roman there who asked, "If I stoke the fire and remove the sponges [that you die more quickly] promise me you will take me with you to heaven." Rabi Hanina agreed, and the Roman did as he said, and promptly jumped into the fire as well. A voice from heaven proclaimed that both Rabi Hanina and the Roman were accepted into the world of life. (ie. the world to come)
[The story can be found on Avoda Zara 18a]
I wanted to draw attention to one particular phrase. The word used to say the letters were flying, אותיות פרחות, is the same word used to describe the souls of Bnei Yisrael at Har Sinai, with each commandment, their souls would fly out of their bodies and be returned anew.
The souls of Bnei Yisrael are the letters of the sefer Torah.
Here, the final moment of Rabi Hanina ben Teradyon, he was experiencing the klal of Bnei Yisrael receiving the Torah on Har Sinai.
What does he say? The text is unclear right here, אף אתה פתח פיך ותכנס בך האש one way to read it is that he says: "Even you, open your mouth, and the fire will enter it."
[כי בפיך ובלבבך לעשותו - because it is in your mouth, and in your heart to do it [uphold the Torah]]
Updated to add: I just realized what a level Rabi Hanina ben Teradyon was on.. Not only did he merit to a revellation of Har Sinai, but also a revelation of Gan Eden in this world. Where? In Gan Eden our Torah becomes our clothes (in addition to our food) .. yet Rabi Hanina merited to be clothed in the Torah in this world as well, when the Romans wrapped him in his Torah scroll.