Likutei Moharan 74:14/ליקוטי מוהר"ן ע״ד:י״ד

114. We see from this that Yitzchak is holy judgment, which extends from the element of lovingkindness, as in “Avraham begot Yitzchak,” as explained above.

2And this is the meaning of Rashi’s comment on this verse: because the scoffers of the generation were saying “Sarah is pregnant by Avimelekh”—i.e., the element of impurity. In other words, the judgment associated with Yitzchak, who was born to Sarah, extends from the element of impurity, God forbid, namely Avimelekh. Therefore, the Holy One formed the features of Yitzchak’s face similar to Avraham’s, so that everyone admitted that “Avraham begot Yitzchak.” The explanation is: The judgment is holy judgment; Yitzchak was born from the element of Avraham, the aspect of lovingkindness, as explained above.