In Avoda Zarah 18a:
The Executioner then said to him (Rabbi Chanina ben Tradyon), 'Rabbi, if I raise the flame and take away the tufts of wool from over your heart, will you bring me into the world to come?' 'Yes,' he replied. 'Then swear unto me' [he urged]. He swore unto him. He thereupon raised the flame and removed the tufts of wool from over his heart, and his soul departed speedily. The Executioner then jumped and threw himself into the fire.
How could Rabbi Chanina ben Tradyon promise someone Olam Haba and furthermore with an oath?