parshanut torah comment - Must every sidrah begin at the start of a parsha? (Rashi on Breishit 47:28)

Rashi on Genesis 47:28 (excerpt)

ויחי יעקב לָמָּה פָּרָשָׁה זוֹ סְתוּמָה?

ויחי יעקב AND JACOB LIVED — Why is this section (weekly reading) totally closed?

Explanation for novices: The Torah scroll is written using paragraphs, which in this terminology is called a parsha. Common terminology uses the term "parsha' to refer to the weekly Torah reading. This is the term Rash"i is using, here.

Usually, the weekly Torah reading begins at the start of a new paragraph. Parshat Vayechi is the only exception that does not. In the Torah, all you see is a space of one letter between the end of the previous weekly reading (Vayigash) and the start of Vayechi.

I can understand that, perhaps, Rash"i is seeing a common pattern that all other readings began at the start of a paragraph, and this seems unusual. However, Rash"i feels a need to explain this anomaly.

Perhaps the incentive to decide where the weekly reading ends happens to make sense to complete a story, law, or some other meaningful unit, and it just happens to end at a paragraph. (I.e., it's incidental.) Maybe, this location was the most logical meaningful place to end Vayigash. Who or what dictated the notion that a weekly reading must end at the end of a paragraph (or that a reading should start at the beginning of one?)

Answer

Berachot 12a

גמירי כל פרשה דפסקה משה רבינו פסקינן דלא פסקה משה רבינו לא פסקינן

We have a tradition that every section which our master, Moses, has divided off we may divide off, but that which our master, Moses, has not divided off, we may not divide off. (Soncino translation)