אמרין עליו על רבי חנינא בן דוסא שהיה עומד ומתפלל ובא חברבר והכישו, ולא הפסיק את תפילתו. והלכו ומצאו אותו חברבר מת מוטל על פי חורו. אמרו “אי לו לאדם שנשכו חברבר, ואי לו לחברבר שנשך את רבי חנינא בן דוסא!” מה עיסקיה דהדין חברבריא? כד הוות נכית לבר נשא – אין בר נשא קדים למיא חברברא מיית ואין חברברא קדים למיא בר נשא מיית. אמרו לו תלמידיו “רבי לא הרגשת?” אמר להן “יבא עלי ממה שהיה לבי מתכוין בתפילה אם הרגשתי.” אמר רבי יצחק בר אלעזר “ברא לו הקב”ה מעיין תחת כפות רגליו, לקים מה שנאמר ‘רצון יריאיו יעשה ואת שוועת’ ישמע ויושיעם’ (תהילים קמה:כ)”:
They say about Rabbi Chanina ben Dosa that he was standing and davening and a snake came and bit him, and he didn’t stop his prayers. They went and found the snake, dead on its hole.
They said, “Woe to the person who is bitten by a snake, and woe to the snake that bites Rabbi Chanina ben Dosa!”
What is the story with this kind of snake? When it bites a person — if the person reaches water first [after being bitten by this kind of snake], the snake dies; and if the snake reaches water first, the person dies.
He students said to him, “Rebbe, didn’t you feel it?”
He said to them, “It would come upon me [something bad; i.e. Rabbi Chanina ben Dosa made an oath] from how my heart was focused on the tefillah if I felt something.
Rabbi Yitzchaq bar Eliezer said, “The Holy One, blessed is He, created [ex nihilo] for him a wellspring under the palms of his feet, to fulfill what it says, “The will of those who fear / are in awe of Him He does, and to their cries He listens and saves them.” (Tehillim 145:20 [Ashrei])
— Yerushalmi, Berakhos 5:1, pg. 38a
This snake doesn’t sound like a natural phenomenon. I would suggest that this story is told to relay the following message.
The snake, as per the story at the beginning of the Torah, represents the yeitzer hara. The water, as is usual, is Torah.
If the person reaches the Torah first, the person fulfilled the advice written outright in the Bavli (Qiddushin 30b):
If this monstrosity [i.e. the yeitzer hara] encounters you, drag him into the study hall where you will destroy him.
If we translate R’ Chanina ben Dosa’s response literally, without interpolation, he said, “Something from what I thought about during prayer should come upon me if I felt it!” Meaning, he had been momentarily distracted, giving the “snake” an opportunity. But, had it been more than that slight hesitation, the yeitzer hara would have been able to get him. However, since his tefillah was pure, R’ Yitzchaq ben Eliezer says Hashem rewarded him by creating a wellspring of Torah under Rabbi Chanina’s feet, Rabbi Chanina focused on the Torah thoughts that flowed from his prayer, and was immobilized, freed from that momentary thought planted by the yeitzer hara.
Within this understanding of the message of the story, the second part of this warning about how this snake operates is very powerful: If the yeitzer hara reaches the water first, so that it uses selective quoting and manipulation of the Torah to convince the person that his deeds are holy and that he is doing Hashem’s will, then all is lost. Nothing is as dangerous as a “pious” sinner.