Rashi on Genesis 25:6/רש"י על בראשית כ״ה:ו׳

1הפילגשים THE CONCUBINES — The word is written deficient, (without י, but our texts have a י in both places) because he had only one concubine, Hagar, who is identical with Keturah. Wives are those whom a man marries with a marriage-contract (Ketubah): concubines have no marriage contract, as we explain in the Talmud (Sanhedrin 21a) in reference to David’s wives and concubines.

2נתן אברהם מתנות ABRAHAM GAVE GIFTS — Our Rabbis explained (Sanhedrin 91a) that he transmitted to them the names of impure things. Another explanation is: all that he had received as gifts because of Sarah — and other gifts that people had given him — all these he gave to them because he did not wish to derive any benefit from these.