Rashi on Deuteronomy 21:17/רש"י על דברים כ״א:י״ז

1פי שנים A DOUBLE PORTION — i.e. a portion equal to those of two brothers together (Sifrei Devarim 217:2-4).

2בכל אשר ימצא לו [BUT HE SHALL ACKNOWLEDGE THE SON OF THE HATED FOR THE FIRSTBORN, BY GIVING HIM A DOUBLE PORTION] OF ALL THAT SHALL BE FOUND WITH HIM — From here the Rabbis derived the law that the firstborn does not receive a double share of what is due to come after the death of the father (as, e.g., a debt or a legacy that were payable to his father on a certain date, before which, however, the father died — technically termed ראוי), as he does of what is actually held in possession by the father (מוחזק) (Sifrei Devarim 217:6; Bekhorot 51b).