The Gemara relates: When they took Rabbi Akiva out to be executed, it was time for the recitation of Shema. And they were raking his flesh with iron combs, and he was reciting Shema, thereby accepting upon himself the yoke of Heaven. His students said to him: Our teacher, even now, as you suffer, you recite Shema? He said to them: All my days I have been troubled by the verse: With all your soul, meaning: Even if God takes your soul. I said to myself: When will the opportunity be afforded me to fulfill this verse? Now that it has been afforded me, shall I not fulfill it? He prolonged his uttering of the word: One, until his soul left his body as he uttered his final word: One. A voice descended from heaven and said: Happy are you, Rabbi Akiva, that your soul left your body as you uttered: One.

   

Berakhot 61b:9