What is the ruling of Islamic law concerning one who promised Allah not to look at his wife's private parts, believing it to be forbidden, and does this promise forbid what Allah has made lawful, and what is incumbent upon him, and is it permissible for him to look, or should he remain on his promise, and is expiation (kaffarah) obligatory upon him for his ignorance?

It is permissible for spouses to look at the entire body of the other, including the private parts, and this is the view of the majority of jurists.

If a man promised his wife not to look at her private parts and did not utter an oath, he is permitted to look as mentioned, and there is no sin upon him nor expiation, just as he is permitted to uphold his promise.

However, if he swore by one of the names of Allah Almighty or one of His attributes, he may break his oath and offer expiation for an oath (feeding ten poor persons or clothing them or freeing a slave; if unable, then fasting for three days).