Or HaChaim on Genesis 18:14/אור החיים על בראשית י״ח:י״ד

1 למועד אשוב, I will return at the appointed time, etc. The reason that the angel repeated the promise was that Abraham should not think that because G'd had objected to Sarah's laughter G'd would take back His promise. This is why he said when exactly the promise would come true.