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 #3

3- مُحَمَّدُ بْنُ يَحْيَى عَنْ مُحَمَّدِ بْنِ الْحُسَيْنِ عَنْ عَلِيِّ بْنِ الْحَكَمِ عَنِ الْعَلاءِ بْنِ رَزِينٍ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ عَنْ أَبِي عَبْدِ اللهِ (عَلَيْهِ السَّلام) قَالَ الْمُدَبَّرُ مِنَ الثُّلُثِ وَقَالَ لِلرَّجُلِ أَنْ يَرْجِعَ فِي ثُلُثِهِ إِنْ كَانَ أَوْصَى فِي صِحَّةٍ أَوْ مَرَضٍ.

3. Muhammad ibn Yahya has narrated from Muhammad ibn al-Husayn from Ali ibn al-Hakam from al-‘Ala’ ibn Razin from Muhammad ibn Muslim who has narrated the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said that al-Mudabbar is from one-third of the legacy.’ He (the Imam) said, ‘One can change his will about one-third of his legacy regardless of whether one is ill or in good health.’”