Exodus 4:22 – 23
English Standard Version (ESV)
22 Then you shall say to Pharaoh, ‘Thus says the Lord, Israel is my firstborn son, 23 and I say to you, “Let my son go that he may serve me.” If you refuse to let him go, behold, I will kill your firstborn son.’ ”
From BibleGateway.com: http://www.biblegateway.com/passage/?search=exodus+4.22-23&version=ESV