Jami` at-Tirmidhi 2124
'Urwah narrated that :
'Aishah had informed him that Barrirah came to her ('Aishah) seeking her help for her writ of emancipation, and she had not yet paid anything for her writ of emancipation. So 'Aishah said to her: 'Return to your people, and if they agree to me paying for your writ of emancipation and that your Wala will be for me, then I will do so." So Barrirah mentioned that to her people and they refused. They said: "If she wants the reward for (freeing) you while the Wala is for us, then let her do it." So I mentioned that to the Messenger of Allah (S.A.W) and the Messenger of Allah (S.A.W) said: "Buy her, then free her, for the Wala is only for the one who frees." Then the Messenger of Allah (S.A.W) stood and said: "What is the case of people who make conditions that are not in Allah's Book?  Whoever makes a condition that is not in Allah's Book, then it will not be so for him, even if he were to make a condition a hundred times."
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا اللَّيْثُ، عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ، أَنَّ عَائِشَةَ، أَخْبَرَتْهُ أَنَّ بَرِيرَةَ جَاءَتْ تَسْتَعِينُ عَائِشَةَ فِي كِتَابَتِهَا وَلَمْ تَكُنْ قَضَتْ مِنْ كِتَابَتِهَا شَيْئًا فَقَالَتْ لَهَا عَائِشَةُ ارْجِعِي إِلَى أَهْلِكِ فَإِنْ أَحَبُّوا أَنْ أَقْضِيَ عَنْكِ كِتَابَتَكِ وَيَكُونَ لِي وَلاَؤُكِ فَعَلْتُ ‏.‏ فَذَكَرَتْ ذَلِكَ بَرِيرَةُ لأَهْلِهَا فَأَبَوْا وَقَالُوا إِنْ شَاءَتْ أَنْ تَحْتَسِبَ عَلَيْكِ وَيَكُونَ لَنَا وَلاَؤُكِ فَلْتَفْعَلْ ‏.‏ فَذَكَرَتْ ذَلِكَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ لَهَا رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ ابْتَاعِي فَأَعْتِقِي فَإِنَّمَا الْوَلاَءُ لِمَنْ أَعْتَقَ ‏"‏ ثُمَّ قَامَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏"‏ مَا بَالُ أَقْوَامٍ يَشْتَرِطُونَ شُرُوطًا لَيْسَتْ فِي كِتَابِ اللَّهِ مَنِ اشْتَرَطَ شَرْطًا لَيْسَ فِي كِتَابِ اللَّهِ فَلَيْسَ لَهُ وَإِنِ اشْتَرَطَ مِائَةَ مَرَّةٍ ‏"‏ ‏.‏ قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ وَقَدْ رُوِيَ مِنْ غَيْرِ وَجْهٍ عَنْ عَائِشَةَ ‏.‏ وَالْعَمَلُ عَلَى هَذَا عِنْدَ أَهْلِ الْعِلْمِ أَنَّ الْوَلاَءَ لِمَنْ أَعْتَقَ ‏.‏