Studying Under Another Rabbi

Is there, and if so, what is the source for the prevalent opinion that in order to rule on Jewish law for the public, a Rabbi must first 'serve' under another Rabbi, and his Torah proficiency alone is not sufficient?

Dear …!
In all generations, the Torah has been passed from Rabbi to student. Until the destruction of the Second Temple, there was “Smichah” ('ordination'), and even after the ordination ceased, no one taught before receiving 'permission to teach'.
There are two reasons for this. A. It needs to be clarified that he indeed studied properly. B. There are many parts of the Torah that build on the Oral Law, and they remail oral, even after all the books have been written. This can only be passed on from a Rabbi to his student.