Yoma 70A

ובלבד שלא ידלג מסוף הספר לתחילתו.

All this is provided that he does not skip from the end of the book to its beginning, since then it would be clear to everyone that he is skipping text.

וגולל ספר תורה וכו׳. וכל כך למה ⁠— כדי שלא להוציא לעז על ספר תורה.

§ It was taught in the Mishnah: The High Priest furls the Torah scroll and places it on his bosom and says: More than what I have read before you is written here. The Talmud comments: And why must he say all this? It is so as not to cast aspersions on the Torah scroll, because people might think that the portion he read by heart is not written there.

״ובעשור״ של חומש הפקודים קורא על פה. אמאי? נגלול וניקרי! אמר רב הונא בריה דרב יהושע אמר רב ששת: לפי שאין גוללין ספר תורה בציבור, מפני כבוד ציבור.

§ It was further taught in the Mishnah: The Torah portion beginning with the verse: “And on the tenth,” from the book of Numbers (29:7), he reads by heart. The Talmud asks: Why does he read it by heart? Let him furl the scroll to that portion and read it from the text. Rav Huna, son of Rav Yehoshua, said that Rav Sheshet said: It is because one may not furl a Torah scroll in public, out of respect for the community. It is inappropriate to make the community wait until they have reached the next section.

ונייתי אחרינא ונקרי! רב הונא בר יהודה אמר: משום פגמו של ראשון. וריש לקיש אמר: משום ברכה שאינה צריכה.

But why not let another Torah scroll be brought that has previously been furled to that portion and read from it? Rav Huna bar Yehuda said: People might mistakenly think the second scroll was brought due to a flaw that was found in the first one. And Reish Lakish said a different reason: A second scroll should not be brought due to the fact that doing do will cause an unnecessary blessing to be recited; before reading from a new scroll the High Priest would have to repeat the blessings required upon reading from the Torah. Therefore, it is preferable that he read by heart.