Musnad Ahmad 25:1678

Rawh narrated to us, Hammad—meaning Ibn Salamah—narrated to us, ‘Ata’ ibn al-Sa’ib narrated to us, from Sa’id ibn Jubayr, from Ibn ‘Abbas, from the Prophet (may Allah bless him and grant him peace), who said: "The Black Stone is from Paradise, and it was whiter than snow, until the sins of the polytheists blackened it."