Abu Juraiy Al-Hujaimi (May Allah be pleased with him) reported

I saw the Messenger of Allah (ﷺ) and said: "Alaikas-Salamu ya Rasulallah! (Upon you be peace, O Messenger of Allah)!" He said, "Do not say: 'Alaikas-Salamu (Upon you be peace).' This is the Salam to the dead."

[Abu Dawud and At-Tirmidhi, who classified the hadith as Hasan Sahih].