English translation — Likkutei Sichos Volume 1, page 23

8.

One of the virtues to which our father Abraham attained through the commandment of circumcision is,

that then he became worthy to give birth to Isaac,

and according to the statement of our sages, may their memory be blessed¹¹,

that Isaac had to be born specifically from a holy drop,

as it is stated¹²

also the expression "Who sanctified a beloved one from the womb."

Therefore Abraham — although also before the commandment of circumcision he was a chariot to G‑dliness,

but he was still not worthy to have Isaac,

only at the time that he attained to "And you shall be perfect" through circumcision,

then specifically could Isaac's birth be.