Rashi on I Kings
That Dovid slept. For Dovid who died of natural causes, an expression of sleeping [שכיבה] is mentioned; but for Yoav who was killed, an expression of dying [מיתה] is mentioned.. Another explanation is that concerning Dovid who was survived by a meritorious son who took his place, [an expression of] sleeping is mentioned, but concerning Yoav who was not survived by a meritorious son who took his place, [an expression of] sleeping is not mentioned, rather, [an expression of] dying [is mentioned].7Maseches Bava Basra 116a.