Maliks Muwatta Book 38, Hadith Number 13.
Section : Freeing the Living for the Dead.
Malik related to me from Abd ar-Rahman ibn Abi Amra al-Ansari that his mother had wanted to make a bequest, but she delayed until morning and died. She had intended to set someone free, so Abd ar-Rahman said, ‘I said to al-Qasim ibn Muhammad, ‘Will it help her if I free a slave for her?’ Al-Qasim replied, ‘Sad ibn Ubada said to the Messenger of Allah, may Allah bless him and grant him peace, ‘My mother died, will it help her if I set a slave free for her?’ The Messenger of Allah, may Allah bless him and grant him peace, said ‘Yes.'”