John Gill Commentary 1 Samuel 26:17

"And Saul knew David`s voice, and said, Is this thy voice, my son David? And David said, It is my voice, my lord, O king." — 1 Samuel 26:17 (ASV)

And Saul knew David's voice
Though Abner at first did not, as appears by his words, but Saul did, by being this time thoroughly awake through the discourse that passed between David and Abner:

and said, [is] this your voice, my son David ?
the same question he put before, when he followed him out of the cave, (See Gill on 1 Samuel 24:16):

and David said, [it is] my voice, my lord, O king ;
he not only owns him to be king, whom he sought not to depose, but his own liege lord and sovereign, whose commands he was ready to obey.