[2:248] And their prophet said to them, 'The sign of his kingship is that there will come to you the Ark. Therein is a Spirit of Peace from your Lord, and a remnant of what the folk of Moses and the folk of Aaron left behind, the angels bearing it. Surely in that shall be a sign for you if you are believers'.
Verily when God (s) makes a light appear, He increases it with help from Himself. When Saul was made king over them, the doubt about his qualification was eliminated by the appearance of [God's] signs proving the veracity of the words of their prophet in choosing him. He returned the Ark which contained the Spirit of Peace (sakīna) to them. Thus the sign of his kingship became clear to them and the fact that their prophet (ʿa) had spoken truly to them in what he told them.
It is said that God Most High placed the Spirit of Peace (sakīna) of the Children of Israel in the Ark which contained the shards of the tablets and the rod of Moses (ʿa), and [other] relics of their possessors of prophecy. He placed the Spirit of Peace of [Muḥammad's] community in their hearts: He it is Who sent down the spirit of Peace into the hearts of the believers [48:4]. Then verily the Ark circulated in the hands of enemies and others. One time it was buried, another time it was seized and carried away, another time it was returned, and another time [and so on...]. As for the hearts of the believers, He comes between their possessors and [the Spirit of Peace]. He did not entrust it to a king or a prophet, or to the heavens or air, or to a place or person. The Prophet (ṣ) said, 'The heart of the believer is between the two fingers of the Merciful', meaning in the grasp of the Real (s) and under His mastery (taglīb) and disposition (taṣrīf). What is meant by it is the divine power of deliberate action (qudra). What a difference between a community whose Spirit of Peace is in what enemies can control and a community whose Spirit of Peace is in that over which no created being has any control!