Al-Kāfi - Volume 4 > The Beginning of the Stone (black stone) and the Reason for Kissing This Stone
Hadith #2

2ـ عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ سَهْلِ بْنِ زِيَادٍ عَنْ أَحْمَدَ بْنِ مُحَمَّدِ بْنِ أَبِي نَصْرٍ عَنْ عَبْدِ الله بْنِ بُكَيْرٍ عَنِ الْحَلَبِيِّ قَالَ قُلْتُ لأَبِي عَبْدِ الله (عَلَيْهِ السَّلاَم) لِمَ جُعِلَ اسْتِلامُ الْحَجَرِ فَقَالَ إِنَّ الله عَزَّ وَجَلَّ حَيْثُ أَخَذَ مِيثَاقَ بَنِي آدَمَ دَعَا الْحَجَرَ مِنَ الْجَنَّةِ فَأَمَرَهُ فَالْتَقَمَ الْمِيثَاقَ فَهُوَ يَشْهَدُ لِمَنْ وَافَاهُ بِالْمُوَافَاةِ.

2. A number of our people have narrated from Sahl ibn Ziyad from Ahmad ibn Muhammad ibn abu Nasr from ‘Abd Allah ibn Bukayr from al-Halabiy who has said the following: “I once asked abu ‘Abd Allah, ‘Alayhi al-Salam, ‘Why should one touch (kiss) the Stone?’ He (the Imam) replied, ‘When Allah, the Most Majestic, the Most Glorious, made a covenant with the children of Adam, He called the Stone from paradise and gave it a command. It swallowed up (fed it to itself) the covenant and it will testify in favor of those who comply with safe return of the trust.’”