John Gill Commentary 1 Samuel 26:9

"And David said to Abishai, Destroy him not; for who can put forth his hand against Jehovah`s anointed, and be guiltless?" — 1 Samuel 26:9 (ASV)

And David said to Abishai, destroy him not
He laid his commands upon him not to hurt him:

for who can stretch forth his hand against the Lord's anointed, and be
guiltless ?
since Saul was king, and appointed to that office by the Lord, and was anointed by his order for it, and invested with it by him, his person was sacred, and not to be touched; nor could his life be taken away by any without being guilty of a very great crime indeed, which it might be justly expected the Lord would resent and punish.