The Twenty-Four Books and the One Shepherd in Tanchuma

Another interpretation: "And like implanted nails" (Ecclesiastes 12:11). From here the Sages said: It is forbidden to read in the outside books, as it is stated, "like implanted nails" (msmrot). Just as the watches (mishmarot) of the priests are twenty-four, so too the books are twenty-four. "The masters of assemblies" (ibid.) — these are the Sanhedrin. And if you should say: This one permits and that one forbids, and this one declares unfit and that one declares fit, and this one declares unclean and that one declares clean — Rabbi Eliezer obligates and Rabbi Yehoshua exempts, the House of Shammai forbid and the House of Hillel permit — to whom shall I listen? Even so, "they were all given from one shepherd" (ibid.).