Genesis 21:12

21:12 But God said to Abraham, “Do not be upset ¹  about the boy or your slave wife. Do ²  all that Sarah is telling ³  you because through Isaac your descendants will be counted. ⁴ 

Hebrews 11:18

11:18 God had told him, “Through Isaac descendants will carry on your name,” ⁵ 

[21:12]  ¹ tn Heb “Let it not be evil in your eyes.”

[21:12]  ² tn Heb “listen to her voice.” The idiomatic expression means “obey; comply.” Here her advice, though harsh, is necessary and conforms to the will of God. Later (see Gen 25), when Abraham has other sons, he sends them all away as well.

[21:12]  ³ tn The imperfect verbal form here draws attention to an action that is underway.

[21:12]  ⁴ tn Or perhaps “will be named”; Heb “for in Isaac offspring will be called to you.” The exact meaning of the statement is not clear, but it does indicate that God’s covenantal promises to Abraham will be realized through Isaac, not Ishmael.

[11:18]  ⁵ tn Grk “in Isaac seed will be named for you.”