Midrasz do Diwrej ha-jamim I 7:40

R. Chanina b. Antignos said: "Also one who was in the records of the kingly guards." R. Juda said in the name of Samuel: "This refers to the bodyguards of David." R. Joseph said: "What is the passage to prove this? (I Chronicles 7, 40) And being recorded according to their genealogy for the army and for the war." Why were they so investigated before they joined David's army? Said R. Juda in the name of Rab: "So that the merits of their ancestors should help them in the war." But there is also (II Sam. 23, 37) Zelek, the Ammonite. Does this not mean a real Ammonite [hence he was not of a distinguished family]? No, this means that he was living in Ammon. But again it is mentioned (Ib.) Uriah, the Hittie. Does it not mean who was a descendant of the tribe of Heth? No, this means that he was living in Heth. And again there is mentioned Itthai, the Gitthaite; and if you will say that this also means that he was living in Gath, did not R. Nachman say, "Itthai the Gitthai came and destroyed [the idol to which the kings would serve]?" Again R. Juda said in the name of Rab: "David had four hundred young children all of whom were the descendants of women who were taken in the war, whose hair was cut loose upon their forehead and their long flowing curls behind. And they used to ride in chariots of gold at the head of the army, who were called men of power, the mighty men of the house of David." These were merely to bring terror into the world. [However, they did not go into war].