Reference for Shevuot 7:12
אמר רב אשי אמריתא לשמעתא קמיה דרב כהנא ואמר לי לא תימא רבי נסיב לה אליבא דתנאי וליה לא ס"ל
- Hence, said R'Joseph, we must conclude that the Tanna of the Mishnah is Rabbi himself, who [as editor] incorporates the views of both Tannaim; for the laws of uncleanness he gives the view of R'Ishmael, and for the laws of oaths he gives the view of R'Akiba [the Mishnah referring accordingly to unwitting transgression]. Said R'Ashi: I repeated this statement [of R'Joseph's] to R'Kahana; and he said to me: Do not think that [R'Joseph meant that] Rabbi simply incorporated in the Mishnah the views of both Tannaim, he himself not agreeing; but the fact is that Rabbi himself, for a sufficiently good reason, agrees [with R'Ishmael in the laws of uncleanness and with R'Akiba in the laws of oaths].