Isaac prayed to the Lord on behalf of his wife, because she was unable to have children; and the Lord answered him, and his wife Rebekah conceived.
Genesis 25:21 (NASB)
The rabbis note that the first part of Genesis 25:21 is written backwards. Normally, it would note the ailment (the barrenness), and then follow it with prayer.
But here, Isaac prays for Rebekah, and then the text lists the ailment. The teaching: prayer is primary. Always pray.