Then Abraham fell upon his face, and {f} laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?

(f) Which proceeded from a sudden joy, and not from lack of faith.

Wesley's Notes

17:17 Then Abraham fell on his face, and laughed - It was a laughter of delight, not of distrust. Now it was that Abraham rejoiced to see Christ's day, now he saw it and was glad, John 8:56, for as he saw heaven in the promise of Canaan, so he saw Christ in the promise of Isaac, and said, Shall a child be born to him that is an hundred years old? - He doth not here speak of it, as at all doubtful, for we are sure he staggered not at the promise, Rom 4:20, but as wonderful, and that which could not be effected but by the almighty power of God.

Jamieson-Fausset-Brown Bible Commentary

17. Abraham fell upon his face, and laughed-It was not the sneer of unbelief, but a smile of delight at the improbability of the event (Ro 4:20).

Matthew Henry's Concise Commentary

17:15-22 Here is the promise made to Abraham of a son by Sarai, in whom the promise made to him should be fulfilled. The assurance of this promise was the change of Sarai's name into Sarah. Sarai signifies my princess, as if her honour were confined to one family only; Sarah signifies a princess. The more favours God confers upon us, the more low we should be in our own eyes. Abraham showed great joy; he laughed, it was a laughter of delight, not of distrust. Now it was that Abraham rejoiced to see Christ's day; now he saw it and was glad, Joh 8:56. Abraham, dreading lest Ishmael should be abandoned and forsaken of God, put up a petition on his behalf. God gives us leave in prayer to be particular in making known our requests. Whatever is our care and fear, should be spread before God in prayer. It is the duty of parents to pray for their children, and the great thing we should desire is, that they may be kept in covenant with Him, and may have grace to walk before him in uprightness. Common blessings are secured to Ishmael. Outward good things are often given to those children of godly parents who are born after the flesh, for their parents' sake. Covenant blessings are reserved for Isaac, and appropriated to him.