Parshas Behar: Alexander Rebbe - The Crime Of Appearing Religious
One time a Yid came to the Alexander Rebbe. This Yid who was not a particularly good person and took mitzvos very lightly, but always showed himself to be a very pious Jew. The pasuk says (Behar 25:17), "V'lo Sonu Ish Es Amiso V'Yareisa Meilokecha; Do not pain or trick your friend and be fearful of Hashem." Why does it says to be fearful of Hashem by this particular mitzva?
The Rebbe said to this person that V'Yareisa Meilokecha teaches us that not only tricking your friend monrtarily or insulting him is assur, but even when it comes to giving an appearance of being a Yiras Shamayim, the pasuk says that it is assur to try and trick others. (Yagdil Torah)