[car.insur]

157. That they said (in boast),
"We killed Christ Jesus
The son of Mary,
The Apostle of God";—
But they killed him not,
Nor crucified him,
But so it was made
To appear to them,
And those who differ
Therein are full of doubts,
With no (certain) knowledge,
But only conjecture to follow,
For of a surety
They killed him not:—

The text doesn't actually state Jesus was substituted - this is in the commentaries -- an opinion which may or may not be right. It could be Jesus (as) was nailed to the cross but didn't die or maybe the whole event was an illusion to those present? There is no Hadith giving more information on the matter and this is the only verse in the Qur'an. Allah (swt) didn't see it fit to tell us what exactly happened except Jesus (as) didn't die and was raised up (4:158).