Sound; and al-Tirmidhi said, “A good, sound hadith.”

Explanation:
The author—may Allah have mercy on him—cites in Riyad al-Salihin, in the Book of Dress, the narration of Jabir ibn Sulaym (may Allah be pleased with him). He had come to Madinah and saw a man whose opinion everyone deferred to; they would not speak until they had taken their lead from him. This shows that they accepted whatever he said and directed because he was the Messenger of Allah, may Allah bless him and grant him peace. Being a newcomer who did not recognize the Prophet ﷺ, Jabir asked, “Who is this?” They answered, “The Messenger of Allah.” He approached and said, “Are you the Messenger of Allah?” The Prophet replied, “Yes.” Jabir then greeted him with ʿalayka al-salām (“upon you be peace”), putting the object first. The Prophet ﷺ said, “Do not say ʿalayka al-salām; ʿalayka al-salām is the greeting of the dead. Instead, say al-salām ʿalayka (peace be upon you).”

His statement, “ʿalayka al-salām is the greeting of the dead,” means that in pre-Islamic times people would greet the dead in this manner, as the poet said:

ʿAlayka salām-u-llāhi, Qays ibn ʿĀmir,
And His mercy for as long as He wills to show mercy.

Thus, in the Days of Ignorance, whenever they greeted the dead they would say ʿalayka