וְהִנֵּ֨ה דְבַר־יְהוָ֤ה אֵלָיו֙ לֵאמֹ֔ר לֹ֥א יִֽירָשְׁךָ֖ זֶ֑ה כִּי־אִם֙ אֲשֶׁ֣ר יֵצֵ֣א מִמֵּעֶ֔יךָ ה֖וּא יִֽירָשֶֽׁךָ׃
And, behold, the word of the LORD came unto him, saying: ‘This man shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir.’
Read halakhic rulings (Jewish law) on Genesis 15:4: and classical halakhic sources.