Musar zu Schemot 23:26

As stated, the three letters in צחק allude to the respective ages of Sarah (90) and Abraham (100) when Isaac was born; the letter ח in the middle alludes to the number (8), the day on which Isaac was circumcised. When we write the word in מלואים, i.e. צדיק-חת-קוף, we have an allusion to the three gifts i.e תורה-ארץ ישראל-עולם הבא. Abraham and Sarah were צדיקים. He converted the men, she the women.