Sahih Bukhari Volume 3, Book 50, Hadith 2561

وَقَالَ اللَّيْثُ حَدَّثَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ عُرْوَةُ قَالَتْ عَائِشَةُ ـ رضى الله عنها ـ إِنَّ بَرِيرَةَ دَخَلَتْ عَلَيْهَا تَسْتَعِينُهَا فِي كِتَابَتِهَا وَعَلَيْهَا خَمْسَةُ أَوَاقٍ، نُجِّمَتْ عَلَيْهَا فِي خَمْسِ سِنِينَ، فَقَالَتْ لَهَا عَائِشَةُ وَنَفِسَتْ فِيهَا أَرَأَيْتِ إِنْ عَدَدْتُ لَهُمْ عَدَّةً وَاحِدَةً، أَيَبِيعُكِ أَهْلُكِ، فَأُعْتِقَكِ، فَيَكُونَ وَلاَؤُكِ لِي فَذَهَبَتْ بَرِيرَةُ إِلَى أَهْلِهَا، فَعَرَضَتْ ذَلِكَ عَلَيْهِمْ فَقَالُوا لاَ إِلاَّ أَنْ يَكُونَ لَنَا الْوَلاَءُ‏.‏ قَالَتْ عَائِشَةُ فَدَخَلْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَذَكَرْتُ ذَلِكَ لَهُ‏.‏ فَقَالَ لَهَا رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ اشْتَرِيهَا فَأَعْتِقِيهَا، فَإِنَّمَا الْوَلاَءُ لِمَنْ أَعْتَقَ ‏"‏‏.‏ ثُمَّ قَامَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏"‏ مَا بَالُ رِجَالٍ يَشْتَرِطُونَ شُرُوطًا لَيْسَتْ فِي كِتَابِ اللَّهِ، مَنِ اشْتَرَطَ شَرْطًا لَيْسَ فِي كِتَابِ اللَّهِ فَهْوَ بَاطِلٌ، شَرْطُ اللَّهِ أَحَقُّ وَأَوْثَقُ ‏"‏‏.‏

Narrated Aisha that Burira came to seek her help in her writing of emancipation snd she had to pay five Uqiyas(of gold) by five yearly instalments.Aisha said to her "do you think that if i pay the whole sum at once, your masters will sell you to me, and i will free you and your wala will be for me."Burira went to her masters and told them about that offer.They said that they would not agree to it unless her Wala will be for them.Aisha further said"I went to Allah's Apostle(s.a.w) and told him about it".Allah's Apostle(s.a.w.) said to her" Buy Burira and manumit her and the wala be for liberator".Allah's Apostle(s.a.w.) then got up and said"What about those people who stipulate conditions that are not present in Allah's laws.If anybody stipulates condition which is not in Allah's laws then what he stipulates is invalid.Allah's conditions(laws) are the truth and are more solid".