Maliks Muwatta Book 38, Hadith Number 18.

Malik related to me from Nafi from Abdullah ibn Umar that A’isha umm al-muminin wanted to buy a slave-girl and set her free. Her people said, “We will sell her to you provided that her wala’ is ours.” She mentioned that to the Messenger of Allah, may Allah bless him and grant him peace, and he said, “Don’t let that hinder you, for the wala’ only belongs to the one who sets free.”