מדרש על בראשית 16:13

(Fol. 4b) MISHNA IX: He (R. Akiba) was wont to say: "The father influences his son in beauty, force, wealth, wisdom, longevity, and the reward to be bestowed on [his] posterity; and herein lies the end of destiny, as it is said (Is. 41, 4) He predetermines from the beginning of fate of the generations to come, (Fol. 5) and though it is said (Gen. 16, 13) They will enslave them and torture them for four hundred years, yet there is another passage (Ib. 16) The fourth generation will return again unto here."

Our sages, however, maintained that she was actually a different wife. Rabbi insisted: Hagar and Keturah were one and the same person, since it is written about Isaac: And Isaac came from the way of Beer-lahai-roi (Gen. 24:62); that is, from the well of which it is written: And she called the name of the Lord that spoke to her, Thou art a God of seeing (ibid. 16:13). From this you learn that she must have been Hagar.⁹A well opened for Hagar after she and Ishmael were sent away (Gen. 21:19). God saw her plight.

We are taught in another Baraitha that R. Eliezer says: "The Messianic period will be forty years," as it is written here (Deut. 8, 3) And he afflicted thee, and suffered thee to hunger, and it is written there (Ps. 90, 15) Cause us to rejoice as many days as those wherein Thou hast afflicted us." [Just as their journey in the desert was forty years, so long will be the days of the Messiah.] R. Dosa, however, says: "Four hundred years, as it is said Cause us to rejoice as many days as thou hast afflicted us, and it is also written (Gen. 15, 13) And they will afflict them four hundred years." Rabbi said: "Three hundred and sixty-five years, according to the days of the Solar year, as it is said (Isa. 63, 4) For the days of vengeance was in my heart, and the year of My redeemer was come." What does the day of vengeance is in My heart mean? R. Jochanan said: "I revealed it to My heart, but not to any other member of My body." And R. Simon b. Lakish said: "I revealed it to My heart, but not to the ministering angels." Abimi b. Obahu taught: "Seven thousand years will be the Messianic period for Israel, as it is said (Ib. 62, 5) And as a bridegroom is glad over the bride, so will thy God be glad over thee." R. Juda said in the name of Samuel: "The Messianic period will be as long as it is from the day of creation till now," as it is said (Deut. 11, 21) As the days of heaven over the earth." R. Nachman b. Isaac said: "As from the day of Noah till now," as it is said (Is. 54, 9) For as the waters of Noah is this unto Him! as I have sworn, etc."