Al-Kāfi - Volume 7 > Al-Mudabbar (the Slave for whose Freedom Death of the Master is a Condition) is from One Third of the Legacy
Hadith #4

4- عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ وَمُحَمَّدُ بْنُ إِسْمَاعِيلَ عَنِ الْفَضْلِ بْنِ شَاذَانَ جَمِيعاً عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ مُعَاوِيَةَ بْنِ عَمَّارٍ قَالَ سَأَلْتُ أَبَا عَبْدِ اللهِ (عَلَيْهِ السَّلام) عَنِ الْمُدَبَّرِ قَالَ هُوَ بِمَنْزِلَةِ الْوَصِيَّةِ يَرْجِعُ فِيمَا شَاءَ مِنْهَا.

4. Ali ibn Ibrahim has narrated from his father and Muhammad ibn ’Isma‘il has narrated from al-Fadl ibn Shadhan from all from ibn abu ‘Umayr from Mu‘awiyah ibn ‘Ammar who has narrated the following: “I once asked abu ‘Abd Allah, ‘Alayhi al-Salam, about al-Mudabbar. He (the Imam) said, ‘It is like will which one can change in whatever way one likes.’”