Rashi on Deuteronomy 15:18/רש"י על דברים ט״ו:י״ח

1כי משנה שכר שכיר [IT SHALL NOT SEEM HARD UNTO THEE WHEN THOU LETTEST HIM GO FREE …] FOR [HE HATH BEEN WORTH] A DOUBLE HIRED SERVANT [UNTO THEE] — From here they (the Rabbis) derived the law that a Hebrew servant has to do service both by day and by night, and that is double as much as the labor of a man hired for day work only. And what is his service during the night? That his master gives him a Canaanite maid-servant with the object of raising children, and the children belong to the master (Sifrei Devarim 123:1; Kiddushin 15a).