John Calvin Commentary Genesis 24:15

"And it came to pass, before he had done speaking, that, behold, Rebekah came out, who was born to Bethuel the son of Milcah, the wife of Nahor, Abraham`s brother, with her pitcher upon her shoulder." — Genesis 24:15 (ASV)

Before he had done speaking. The outcome sufficiently demonstrates that his wish had not been foolishly conceived. For the swiftness of the answer reveals the extraordinary favor of God, who does not allow the man to be long troubled with anxiety. Rebekah had indeed left her house before he began to pray; yet it must be maintained that the Lord, who controls both the moments of time and the ways of humanity, had so ordered it on both sides as to give a clear demonstration of his providence.

For sometimes he keeps us in suspense longer, until, weary from praying, we may seem to have labored in vain. But in this matter, so that his blessing might not seem uncertain, he suddenly intervened. The same thing also happened to Daniel, to whom the angel appeared before he finished his prayer (Daniel 9:21). Now, although it frequently happens that because of our sloth, the Lord delays granting our requests, it is, at such times, beneficial for us that what we ask for is delayed. Meanwhile, he has openly and clearly proved, by undeniable examples, that even if the outcome does not immediately match our wishes, the prayers of his people are never in vain. Indeed, his own declaration, that before they cry he is mindful of their wants, is invariably fulfilled (Isaiah 65:24).