As we start Sefer Vayikra, my chavruta and I have again looked for and been unable to find a satisfactory answer to the question: “What is the origin of the division of the Torah into five books and how ancient is the division - when is it first mentioned in halachic works?”
1 Answer
It’s mentioned in the Gemara Gittin 60A (there may be earlier sources, this is just one I know of):
שְׁלַחוּ לֵיהּ בְּנֵי גָלִיל לְרַבִּי חֶלְבּוֹ: מַהוּ לִקְרוֹת בְּחוּמָּשִׁים בְּבֵית הַכְּנֶסֶת בְּצִיבּוּר?
The people of the Galilee sent a question to Rabbi Ḥelbo: What is the halakha with regard to reading from ḥumashim, i.e., scrolls containing only one of the five books of the Torah, in the synagogue in public?
Rashi there explains that it refers to one of the five chumashim, meaning they had this split in the times of the Gemara at least:
בחומשין - שיש שכותבין להן חמשה חומשין כל חומש אחד שלם לעצמו וכל ספריהם היו במגילה כס"ת שלנו
Edit:
According to the Midrash Tehillim 1;2 Dovid Hamelech made Tehillim five books to correspond to the five Chumashim. Furthermore, it indicates that Moshe Rabbeinu himself split it into five
משה נתן להם חמשה חומשי תורה. וכנגדם נתן להם דוד ספר תהלים שיש בו חמשה ספרים
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1The word חומשים literally means "fifths". The singular is חומש a fifth, like the phrase קרן וחומש– Double AA ♦Commented Mar 18 at 0:48
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@shmosel חוּמש isn't a word to the best of my knowledge. The plural of חוֹמֶשׁ is חוּמָשִׁים and people who don't know better truncate חוּמָשִׁים to just חוּמָשׁ without realizing the vowel should re-lengthen. (Like the vowels in the familiar stitch לְמקְדְּשֵךְ תּוּב וּלְקוֹדֶשׁ קוּדְשִׁין)– Double AA ♦Commented Mar 18 at 0:54