Albert Barnes Commentary 1 Kings 1:52

"And Solomon said, If he shall show himself a worthy man, there shall not a hair of him fall to the earth; but if wickedness be found in him, he shall die." — 1 Kings 1:52 (ASV)

There shalt not an hair ... - This was a proverbial expression, meaning “he shall suffer no hurt at all.” Solomon’s clemency in pardoning Adonijah is very remarkable. In the East, not only are pretenders almost always punished with death, but it has often been the custom for each king upon his accession to put to death all his brothers as mere possible pretenders.