I will surely return to you about this time next year, and Sarah your wife will have a son

 Genesis 18:1-19

Abraham looked up and saw three men standing nearby. When he saw them, he hurried from the entrance of his tent to meet them and bowed low to the ground.

He said, “If I have found favor in your eyes, my lord, do not pass your servant by. Let a little water be brought, and then you may all wash your feet and rest under this tree. Let me get you something to eat, so you can be refreshed and then go on your way—now that you have come to your servant.”

“Where is your wife Sarah?” they asked him. “There, in the tent,” he said. Then one of them said, “I will surely return to you about this time next year, and Sarah your wife will have a son.”

So Sarah laughed to herself as she thought, “After I am worn out and my lord is old, will I now have this pleasure?”

Sarah was afraid, so she lied and said, “I did not laugh.” But he said, “Yes, you did laugh.”

Why?
Treat strangers as you treat God's angels. So Abraham served these three men, who were angels.

No matter what is possible or impossible, it is in the hands of the Lord. 

We thought it impossible, but in the hands of the Lord could be possible if God wills.

How?

What I can do if the Lord walks with me, I can do anything, but if the Lord doesn't, I can do nothing.