The Dubno Magid zt”l once visited a rich man named Yitzchok who was known to be very stingy. He asked the man to donate to a charitable cause, but he refused to give anything. The Magid then told him, “If you don’t give anything, everyone will say that you are a mamzer.”
He quoting Rashi’s words that the scoffers claimed that Yitzchok was really Avimelech’s son. To counter this claim, Hashem made Yitzchok look exactly like Avrohom. Chazal add ( Bava Kama 97B, see Rashi ibid) that Avrohom engraved his face on one side of a coin, and Yitzchok’s face on the other. Thus, even those who had never seen Avrohom could tell that he and Yitzchok were father and son. explained himself by of the generation
The Magid concluded, “We see that Yitzchok can only prove that he was Avrohom’s son if he takes a coin out of his pocket. If he doesn’t, people will say that he is a mamzer.”