Sahih Bukhari Volume 8, Book 85, Hadith 6759

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ عَائِشَةَ أُمَّ الْمُؤْمِنِينَ، أَرَادَتْ أَنْ تَشْتَرِيَ جَارِيَةً تُعْتِقُهَا فَقَالَ أَهْلُهَا نَبِيعُكِهَا عَلَى أَنَّ وَلاَءَهَا لَنَا‏.‏ فَذَكَرَتْ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏"‏ لاَ يَمْنَعُكِ ذَلِكِ، فَإِنَّمَا الْوَلاَءُ لِمَنْ أَعْتَقَ ‏"‏‏.‏

Narrated By Ibn Umar : That 'Aisha, the mother of the Believers, intended to buy a slave girl in order to manumit her. The slave girl's master said, "We are ready to sell her to you on the condition that her Wala should be for us." 'Aisha mentioned that to Allah's Apostle who said, "This (condition) should not prevent you from buying her, for the Wala is for the one who manumits (the slave)."