Genesis 21.12
Septuagint

eipe de o qeoV tw Abraam, mh sklhron estw enantion sou peri tou paidiou, kai peri thV paidiskhV:  panta osa an eiph soi Sarra, akoue thV fwnhV authV oti 
en Isaak klhqhsetai soi sperma
 

New Testament - Romans 9.7

oud oti eisin sperma Abraam panteV tekna, all : 
en Isaak klhqhsetia soi sperma

Hebrews 11.18

proV on elalhqh oti 
en Isaak klhqhsetai soi sperma

Septuagint

But God said to Abraam, let it not be hard before thee concerning the child, and concerning the bondwoman; in all things whatsoever Sarrha shall say to thee, hear her voice, for in Isaac shall thy seed be called

New Testament

Romans 9.7:  neither, because they are Abrahamís seed, are they all children:  but, In Isaac shall thy seed be called

Hebrews 11.18:  even he to whom it was said, In Isaac shall thy seed be called

Masoretic Text

And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy handmaid; in all that Sarah saith unto thee, hearken unto her voice; for in Isaac shall thy seed be called

Comments:  The NT, LXX and MT agree.