וְדָוִ֖ד ה֣וּא הַקָּטָ֑ן וּשְׁלֹשָׁה֙ הַגְּדֹלִ֔ים הָלְכ֖וּ אַחֲרֵ֥י שָׁאֽוּל׃ (ס)
And David was the youngest; and the three eldest followed Saul.—
Rashi on I Samuel
Dovid was the youngest. Who had previously been in Shaul's palace to play music before him.