Reference for Bava Batra 37:4
וסד בסיד: איבעיא להו וסד בסיד תנן או דילמא או סד בסיד תנן
We find this also taught [in a Baraitha]: The fuller's pool must be kept four cubits away. But did we not learn, THREE HANDBREADTHS? — This shows [that the Mishnah must be understood] as R. Nahman has explained. Some put this statement in the form of a contradiction [which is afterwards reconciled,] thus: We learnt A FULLER'S POOL MUST BE KEPT THREE HANDBREADTHS AWAY. But does not a Baraitha say four cubits? — Said R. Nahman in the name of Rabbah b. Abbuha: There is no contradiction. The Mishnah speaks of the soaking pool and the Baraitha of the washing pool. R. Hiyya the son of R. Awia added a gloss to this effect to the Mishnah itself: 'Unless he removes the edge of the soaking pool three handbreadths from the wall.'