Genesis 20:3

But God came to Abimelech in a dream by night, and said to him, Behold, thou art but a dead man, for the woman which thou hast taken; for she is a man's wife.

God came to Abimelech in a dream by night. In early times a dream was often made the medium of communicating important truths; and this method was adopted for the preservation of Sarah. This is the first instance that has occurred in the course of the sacred history, of God giving a special revelation to any one who was not within the pale of the church and covenant; and therefore it is proper to observe that in all such supernatural communications it was to persons of power and influence, as Joseph, Pharaoh, Nebuchadnezzar, and Daniel, with a prospective reference to the interests of his own people. So it was also in the instance before us.

And said to him, Behold, thou art ... a dead man - [Hebrew hinªkaa (H2009) meet (H4191).] Behold thee about to die. Laid upon a bed of sickness, in common with several inmates of his palace, by a sudden and violent malady, which apparently threatened to have a fatal issue, he was led to serious reflection. His nights were perturbed by dreams, in which his thoughts naturally took their course, as suggested by the special nature of his disease; and while his mind was thus in a state of preparation, God revealed to him in a dream the cause of all his personal and domestic distress.