2 Samuel 12:15 (BSB)

After Nathan had gone home, the LORD struck the child that Uriah’s wife had borne to David, and he became ill.

From 2 Samuel 12. Also in the ESV.

Commentary on 2 Samuel 12:15

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on 2 Samuel 12:15-25: Nathan, having delivered his message, staid not at court, but went home, probably to pray for David, to whom he had been preaching. God, in making use of him as an instrument to bring David to repentance, and as the herald both of mercy and judgment, put an honour upon the ministry, and magnified his word above all his name.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on 2 Samuel 12:15: David therefore besought God for the child,.... Perhaps went into the tabernacle he had built for the ark, and prayed to the Lord to restore the child, and spare its life; for though the Lord had said it should die, he might hope that that was a conditional threatening, and that the Lord might be gracious and reverse it, Sa2 12:22, and David fasted: all...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on 2 Samuel 12:15: the Lord struck the child . . . and it was very sick--The first visible chastisement inflicted on David appeared on the person of that child which was the evidence and monument of his guilt. His domestics were surprised at his conduct, and in explanation of its singularity, it is necessary to remark that the custom in the East is to leave the nearest relative...