Genesis 21:15 - When all hope is lost.

And the water was spent in the bottle, and she cast the child under one of the shrubs.

Apparently, Ishmael was overcome with fatigue and thirst and Hagar was preparing for his death. In her despair, Hagar forgot about God's promise to multiply Ishmael seed (Genesis 16:10). Likewise, when the Israelites became overcome with fatigue and thirst in Kadesh, they also forgot about God's promises (Numbers 20:5). In both cases, when all hope was lost and despair had settled in, God came to their rescue.

Genesis 17:20 - Blessed without Knowing.

And as for Ishmael, I have heard thee: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation.

I have heard thee refers to Abraham's intercessory prayer for his thirteen year old son, Ishmael. However, even before Ishmael was born, God had already blessed his seed to multiply exceedingly that it shall not be numbered for multitude (Genesis 16: 10).