Musar sur Les Nombres 35:31

In some instances one scholar may see as many as three distinct negative commandments involving the consumption of חמץ, leavened things, whereas another scholar considers that there is only a single culpability for all of these prohibitions of חמץ, be it before Passover, during Passover, or after Passover. Even such universally known precepts as the recital of קריאת שמע, are subject to debate, one Rabbi claiming it is a Biblical precept, another saying that it is merely a Rabbinic injunction.