Rashi on Genesis 25:24/רש"י על בראשית כ״ה:כ״ד

1וימלאו ימיה AND WHEN HER DAYS TO BEAR WERE FULL — But in the case of Tamar it is written, (37:27) “And it came to pass at the time of her travail”, for her days were not fulfilled as she gave birth to them (her children) at the end of seven months.

2והנה תומם BEHOLD, THERE WERE TWINS — The word for twins is written defective (without א and י) whilst in the case of Tamar it is written תאומים (with א and י); because in the latter case both children proved righteous whilst here one was righteous and the other wicked (Genesis Rabbah 63:8).