In the following ayath in the Quran, one can conclude that Ishmael was the son to be sacrificed
And he [—Abraham—] said [after being saved from the fire]: ‘Verily, I
am going to my Lord. He will guide me!’ ‘My Lord! Grant me (offspring)
from the righteous.’ So We gave him glad tidings of a forbearing boy.
And, when he [his son] was old enough to walk with him, he said: ‘O my
son! I have been seeing in a dream that I am offering you in sacrifice
[to Allah]. So look what you think!’ He said: ‘O my father! Do that
which you are commanded, Insha Allah, you shall find me of the
patient. Then, when they had both submitted themselves [to the will of
Allah], and he had laid him prostrate on his forehead, We called out
to him: ‘O Abraham! You have made the dream a reality’. Verily, thus
do We reward the good-doers. Verily, that indeed was a manifest trial.
And We ransomed him with a great sacrifice [a ram]; And We left for
him [a goodly remembrance] among the later generations. Peace be upon
Abraham! Thus indeed do We reward the good-doers. Verily, he was one
of Our believing slaves. And We gave him the glad tidings of Isaac – a
Prophet from the righteous.
-(37:99-112)
The ending portion of the passage clearly shows the glad tidings of the birth of Isaac hence one can conclude that the verse is narrating about Ishmael.
Though, in bible they are places where it clearly says that Isaac was the son offered for sacrifice, muslims believe that it's because it was tampered. One of the evidenced put forward for this aprt from the holy Quran are the following two verses.
Abraham was eighty-six years old when Hagar bore him Ishmael.
-(Genesis, 16:16)
Abraham was a hundred years old when his son Isaac was born to him.
-(Genesis, 21:5)
