Berachos, Chapter Nine, Mishnah 4. Our Mishnah deals with special prayers that are to be recited upon entering a city and upon leaving it. It should be noted at the outset that the city described here is a city where particularly difficult people and situations are found, requiring special prayers. The average city does not fall into this category, at least not in our times.
The view of the first Tanna:
"HaNichnas liKerach mispallel shtayim" - one who enters a large walled city recites two prayers:
"Achas biknisaso" - one before he enters the city, that everything should go smoothly and that he should not run into difficulties.
"V'achas biytziaso" - one at the time that he leaves the city.
These prayers should not be confused with Tefillas HaDerech, which is recited for the journey itself and is a separate halachah.
The view of Ben Azzai:
"Ben Azzai omer: arba" - in his opinion four prayers are to be recited: "Shtayim biknisaso ushtayim biytziaso" - two at the time of entering the city and two at the time of leaving it.
What is the nature of these prayers?
The Mishnah spells it out: "V'nosen hoda'ah l'she'avar v'tzo'ek l'asid lavo":
"Nosen hoda'ah l'she'avar" - after he has entered the city he gives thanks for having entered safely, and likewise at the time he leaves.
"V'tzo'ek l'asid lavo" - the two prayers before entering and before leaving are a request for help from Hashem to enter the city and to leave it safely.
Some explain that the phrase "nosen hoda'ah l'she'avar v'tzo'ek l'asid lavo" refers specifically to the two prayers that Ben Azzai added to the words of the first Tanna. According to the first Tanna there is one prayer before entering the city and one after leaving it, and Ben Azzai adds two prayers: one after he has entered the city - and that is the "thanks for the past," thanking Hashem for having entered safely; and one before he leaves the city - and that is the "cry for the future." It emerges that the wording of the Mishnah describes the two middle prayers that Ben Azzai added, unlike the first Tanna, according to whom the prayers are recited one before entering and one after leaving.
In summary: This Mishnah deals with the prayers recited upon entering a walled city and upon leaving it. According to the first Tanna two prayers are recited - one before entering and one after leaving; according to Ben Azzai, four - two upon entering and two upon leaving. Their content: thanks for the past and a cry for the future, and some explain that this thanks and cry refer specifically to the two prayers that Ben Azzai added.