Sahih Bukhari Volume 3, Book 34, Hadith Number 364.

Narated By ‘Aisha : Allah’s Apostle came to me and I told him about the slave-girl (Buraira) Allah’s Apostle said, “Buy and manumit her, for the Wala is for the one who manumits.” In the evening the Prophet got up and glorified Allah as He deserved and then said, “Why do some people impose conditions which are not present in Allah’s Book (Laws)? Whoever imposes such a condition as is not in Allah’s Laws, then that condition is invalid even if he imposes one hundred conditions, for Allah’s conditions are more binding and reliable.”