Rashi on Mishpatim 23:21

English: אל תמר בו EXASPERATE HIM NOT — This verb is a term denoting rebellion (המראה), similar to, (Joshua 1:18) “Whosoever he be that rebel (ימרה) against thy commandment”. כי לא ישא לפשעכם FOR HE WILL NOT PARDON YOUR TRESPASS — He is not accustomed to it (to sin) for he is one of a class of beings who never sin. Besides he is only a messenger and only carries out the mission entrusted to him (i. e. he has no right to pardon you, for this is My prerogative: it is his only to insist upon obedience to My commands) (Midrash Tanchuma, Mishpatim 18). כי שמי בקרבו FOR MY NAME IS IN HIM — This passage must be connected with the beginning of the verse: Take heed of him etc. — for (כי) My Name is associated