Reference for Ketubot 3:4

אמר רב יוסף מריה דאברהם תלי תניא בדלא תניא הי תניא והי לא תניא הא תניא והא תניא אלא תלי תניא דמפרש טעמא בדתניא דלא מפרש טעמא

R. Joseph said: Master of Abraham! He [Shmuel] attaches a Mishnah which was taught, to a Mishnah which was not taught! Which was taught and which was not taught? This was taught and this was taught! Rather he attaches a Mishnah, the reason of which was explained, to a Mishnah, the reason of which was not explained.

Jastrow