1 Samuel 16:1, 6-7, 12-13

And the LORD said unto Samuel, How long wilt thou mourn for Saul, seeing I have rejected him from reigning over Israel? fill thine horn with oil, and go, I will send thee to Jesse the Beth-lehemite: for I have provided me a king among his sons.

And it came to pass, when they were come, that he looked on Eliab, and said, Surely the LORD's anointed is before him. But the LORD said unto Samuel, Look not on his countenance, or on the height of his stature; because I have refused him: for the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart.

And he sent, and brought him in. Now he was ruddy, and withal of a beautiful countenance, and goodly to look to. And the LORD said, Arise, anoint him: for this is he. Then Samuel took the horn of oil, and anointed him in the midst of his brethren: and the Spirit of the LORD came upon David from that day forward. So Samuel rose up, and went to Ramah.