وَامْرَأَتُهُ قَآئِمَةٌ فَضَحِكَتْ فَبَشَّرْنَاهَا بِإِسْحَقَ ...

And his wife was standing (there), and she laughed. But We gave her glad tidings of Ishaq,

Then, Sarah laughed in delight of the good news of their destruction. This is because they had caused much corruption and their disbelief was severe. For this reason, she was rewarded with the glad tidings of a son, even after her despair.

Concerning Allah's statement,

... وَمِن وَرَاء إِسْحَقَ يَعْقُوبَ ﴿٧١﴾

and after Ishaq, of Yaqub.

This means that the son that she was going to have would have a son (her grandson) who would succeed him and beget many children. For verily, Yaqub was the son of Ishaq, just as Allah says in Surah Al-Baqarah,

أَمْ كُنتُمْ شُهَدَآءَ إِذْ حَضَرَ يَعْقُوبَ الْمَوْتُ إِذْ قَالَ لِبَنِيهِ مَا تَعْبُدُونَ مِن بَعْدِى قَالُواْ نَعْبُدُ إِلَـهَكَ وَإِلَـهَ آبَآئِكَ إِبْرَهِيمَ وَإِسْمَـعِيلَ وَإِسْحَـقَ إِلَـهًا وَاحِدًا وَنَحْنُ لَهُ مُسْلِمُونَ

Or were you witnesses when death approached Yaqub! When he said unto his sons, "What will you worship after me!''

They said, "We shall worship your God, the God of your fathers, Ibrahim, Ismail, Ishaq, One God, and to him we submit.'' (2:133)

From this point in this verse there is an evidence for those who say that Ismail was the son of Ibrahim who was to be sacrificed. It could not have been Ishaq, because the glad tidings were given that he would have a son born to him named Yaqub. So how could Ibrahim be commanded to sacrifice him when he was a small child and there had not been born to him a child yet, named Yaqub, who was promised!

The promise of Allah is true and there is no breaking of Allah's promise. Therefore, it is not possible that Ibrahim was to sacrifice this child (Ishaq) with the condition being as it was. This makes it clear that Ismail was the son that was to be sacrificed and this is the best, most correct and clearest evidence of that.

And all praise is due to Allah.