The Black Stone descended from Paradise, and it was whiter than milk, but the sins of the descendants of Adam made it black.

Al-Hajr Al-Aswad (The Black Stone)

The Prophet (peace and blessings of Allah be upon him) said: “The black stone descended from paradise, and it was whiter than milk, but the sins of the descendants of Adam made it black.”

Ahmad and Tirmidhi (graded saheeh by Al-Albani)

Suwaid bin Ghafalah said: I saw the Caliph ‘Umar kiss the Black Stone and say: “I know that you are a stone and you cannot cause harm or bring benefit, but I saw [the Prophet Muhammad] Abul-Qasim (ﷺ) show respect to you [by kissing you].” (Bukhari and Muslim)

Zaid bin Aslam said that his father said: “I saw [the Caliph] ‘Umar bin Al-Khattab kissing the Black Stone and he then said to it: ‘Had I not seen Allah’s Messenger kissing you, (i.e., the stone) I would not have kissed you.'” (Bukhari)