TT #255 · Parshas Shemini · Aliyah 7 · Passuk 44
Chabad.org — Vayikra Chapter 11 with Rashi
English: “You shall sanctify yourselves”: whoever sanctifies himself at the time of marital relations merits male children.
Shevuot 18b.
E-Daf.com — Shevuot 18b (Tsurras Hadaf)
Ethical application — The Chazal applies the verse’s wording and structure to establish the stated teaching.
Self-sanctification at marital intimacy is associated in the source with having male children.