GEN1451 … the Rambam observes that it is prohibited for a Jew to be outwardly friendly towards another individual while he inwardly harbors resentment and animosity towards him. “It is prohibited for an individual to conduct himself with words of falseness and enticement, nor shall he be hypocritical, but rather he should appear outwardly as he is within [i.e. his outward relationship towards people should reflect his true inner attitude], and that which he expresses with his lips should be the same as that which he feels within his heart.” Rambam, Mishneh Torah, hilchos De’os 2:6.   The Chafetz Chaim deals with this matter at great length in his Kuntres Sfas Tamim.   He cites the words of Chazal, [our Sages, may their memories be for a blessing – AJL] who include the hypocrites among “three who are despised by the Almighty.” Talmud, Pesachim 113b.   And he cites also the words of Jeremiah, who decries, in no uncertain terms, the individual who Jeremiah 9:7, speaks peaceably to his neighbor with his mouth, but in his heart he lies in wait. The Torah informs us that the brothers of Yosef were unable to speak peaceably with him. [this verse] Whereupon, Rashi observes, “From their fault we learn their praise, for they did not speak hypocritically – one way with their mouth, and another with the earth.” FENDEL 118-9