Reference for Sanhedrin 116:12

כי אתא רב דימי אמר ר' אלעזר אמר ר' חנינא בן נח שייחד שפחה לעבדו ובא עליה נהרג עליה

R. Huna said: A heathen may marry his daughter. But should you ask, If so, why did not Adam marry his daughter? — In order that Cain might marry his sister, that the world might be built up by grace. Others give this version: R. Huna said: A heathen may not marry his daughter; the proof being that Adam did not marry his daughter. But that proof is fallacious: The reason was that Cain should marry his sister, so that the world should be built up by [Adam's] grace.

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