Sahih Bukhari Volume 3, Book 34, Hadith 2170

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ عَائِشَةَ، أُمَّ الْمُؤْمِنِينَ أَرَادَتْ أَنْ تَشْتَرِيَ جَارِيَةً فَتُعْتِقَهَا، فَقَالَ أَهْلُهَا نَبِيعُكِهَا عَلَى أَنَّ وَلاَءَهَا لَنَا‏.‏ فَذَكَرَتْ ذَلِكَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏"‏ لاَ يَمْنَعُكِ ذَلِكَ، فَإِنَّمَا الْوَلاَءُ لِمَنْ أَعْتَقَ ‏"‏‏.‏

Narrated By 'Abdullah bin 'Umar : 'Aisha, (mother of the faithful believers) wanted to buy a slave girl and manumit her, but her masters said that they would sell her only on the condition that her Wala' would be for them. 'Aisha told Allah's Apostle of that. He said, "What they stipulate should not hinder you from buying her, as the Wala' is for the manumitted."