ENOfficial
He reported God's messenger as saying, “The black stone descended from paradise whiter than milk, but the sins of the descendants of Adam made it black." Ahmad and Tirmidhi transmitted it, the latter saying that this is a hasan sahih tradition.
Source: sunnah.com
Other translations
Arabic
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «نَزَلَ الْحَجَرُ الْأَسْوَدُ مِنَ الْجَنَّةِ وَهُوَ أَشَدُّ بَيَاضًا مِنَ اللَّبَنِ فَسَوَّدَتْهُ خَطَايَا بَنِي آدَمَ» . رَوَاهُ أَحْمَدُ وَالتِّرْمِذِيُّ وَقَالَ: هَذَا حَدِيثٌ حَسَنٌ صَحِيح
sunnah.com