Excerpt from 'Hayat Al-Qulub, Vol. 1':
They were four brothers (including Dawud) who was to free people from the oppression of Jalut and his army men. His Shi‘ahs (partisans) Imam knew that Dawud was born already and had also become major. They were seeing him (Dawud) but they (Dawud’s brothers) did not know that he was Dawud despite talking with him. When Talut assembled The Israelites for a fight with Jalut, Dawud’s father accompanied the army of Talut with his four sons. But the brothers were not prepared to take Dawud with them thinking that he was too weak and hence insignificant and so they said that he should be left at home for grazing their sheep. Anyway fighting began between The Israelites and Jalut.
The Israelites were frightened much and so began to demoralize. Dawud’s father returned home and sent Dawud with provisions for his brothers so that their strength might get a boost. Dawud was short stature, having livid eyes and scanty hair. His soul was quite pure and his character clean. Dawud reached when both armies had come face to face and everyone had taken up his position. He passed by a stone in the path. The stone cried loudly, 'O Dawud! Pick me up and kill Jalut with my hit, because I have been created for killing Jalut.' Dawud took up the stone and put it in the bag in which he was putting other stones to be flung with his sling while grazing his sheep.
When he entered the army of The Israelites he came to know that Jalut was proving very hard for his army (of The Israelites). So he said, 'How do you consider him (Jalut) so Big? By Allah, if I see him I will kill him at once.' His word reached the army until Jalut too heard it. He called Dawud and told him, 'O Young man! How much strength have you and how much experience of war that you dare fight with Jalut?' Dawud replied, 'Once a tiger pounced upon my herd of sheep and began to walk away with one of my sheep. I followed that tiger, twisted his neck and released my sheep from his mouth.' Allah had informed Talut through revelation that the man on whose body your armor will fit exactly (as if made only for him) will kill Jalut.
Talut called for his armor and gave it to Dawud to wear. He wore and despite being quite thin physically it fitted him perfectly. Then Talut and The Israelites became fearful and realized his status. Talut said, I hope this young man will kill Jalut. Next day when both the armies readied for confrontation Dawud asked Talut to show him who was Jalut. (People) showed him Jalut. Dawud took out from his bag the stone that he had picked up from the path, put it in his sling and flung it toward Jalut. It hit him between his two eyes and went up to the brain in his head. He fell down from his horse and a word spread in the army that Dawud killed Jalut. People made him (Dawud) their king.
Thereafter nobody obeyed Talut. Allah revealed Zabur (Book of Psalms) to him (Dawud), taught him how to make armors and made iron as soft as wax in his hands. And Allah commanded birds and mountains to sing praise of Allah along with Dawud. Allah gave Dawud such a melodious voice that none before him had ever heard. He also made Dawud perfect in worship. He lived among The Israelites with both Prophethood and terrestrial rulership.