Sunan Abu Dawud
4517
Sahih Maqtu+3

Qatadah transmitted the tradition mentioned above through a chain of narrators like that of Shu'bah. This version adds:Then al-Hasan forgot this tradition, and he used to say: A free man is not to be killed for a slave

Sahih MaqtuAl-Albani
Sahih MaqtuMuhammad Muhyi Al-Din Abdul Hamid