After they had gone, the men came up out of the well, and went and told King David. They said to David, “Arise, and go quickly over the water, for thus and so has Ahithophel counseled against you.”
We must now leave David's enemies pleasing themselves with the thoughts of a sure victory by following Hushai's counsel, and sending a summons, no doubt, to all the tribes of Israel, to come to the general rendezvous at a place appointed, pursuant to that counsel; and we next find David's friends consulting how to get him notice of all this, that he might steer his course accordingly.
Commenting on 2 Samuel 17:15-21
Then David arose, and all the people that were with him, and passed over Jordan,.... Taking the advice of Hushai: by the morning light there lacked not one of them that was not gone over Jordan; they travelled all night, and before it was morning they had all passed the river in boats, or through the fords, and not one was lost in the passage...
And it came to pass, after they were departed, that they came up out of the well, and went and told king David, and said unto David, Arise, and pass quickly over the water: for thus hath Ahithophel counselled against you. (k) That is, to pursue you with all haste.