Rabenu Yonah in sha’ar gemel elaborates on the topic in the beginging as well as it is adressed by the ran in drush 7. Also look at the gemara brachos on daf daled with rebbe tarfon that one who transgresses the command of the rabanun is m’cheuv me’ssa and it it worth looking at rabenu yonah there as well (he is in the rif in brachos).