Sahih al-Bukhari 6757
Narrated 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 Messenger (ﷺ) who said, "This (condition) should not prevent you from 
buying her, for the Wala is for the one who manumits (the slave)."
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 ‏"‏‏.‏