Genesis 21:30
"And he said, For these seven ewe lambs shalt thou take of my hand, that they may be a witness unto me, that I have digged this well."
Cross References (ranked by relevance)
Genesis 31:52
This heap be witness, and this pillar be witness, that I will not pass over this heap to thee, and that thou shalt not pass over this heap and this pillar unto me, for harm.
Joshua 24:27
And Joshua said unto all the people, Behold, this stone shall be a witness unto us; for it hath heard all the words of the LORD which he spake unto us: it shall be therefore a witness unto you, lest ye deny your God.