The Bodhisatta was once a snake-bite doctor. One time when someone was bit by a snake, the Bodhisatta asked his patient whether he wanted the venom extracted with the usual medicine or if he wanted the snake to suck its own venom out of the wound: he chose the latter. The snake was captured and it admitted biting the man, but it refused to suck the venom out. So the Bodhisatta built a fire and told the snake that if it did not take back its venom it would be thrown in and killed. The snake said it would never do so, preferring to die strong than save its life by being weak, and it crawled toward the fire. The Bodhisatta blocked its path and ordered the snake to never harm anyone again. He then set it free and cured the patient with the usual medicine and charms.
In the Lifetime of the Buddha
The snake was an earlier birth of Sariputta, one of the Buddha’s top disciples. One day some lay visitors had brought some meal-cakes to the monastery for the disciples and left some extras for those who were not there at the time. A portion was set aside for a young disciple who resided with Sariputta, but as noon approached (after which time disciples cannot eat solid food) and he had still not returned, these leftover cakes were given to Sariputta. Right after he ate them, the young disciple returned. Feeling bad about eating his food, Sariputta vowed to never eat meal-cakes again.
When the Buddha heard some disciples talking about this, he told them this story so they knew that Sariputta never broke a vow, even in the past when his life was at stake.