Sunan Abi Dawud 24:92

The tradition mentioned above has also been transmitted by Qatadah through a different chain of narrators to the same effect. This version adds:

"If he finds defect (in the slave) within three days, he may return it without evidence; if he finds a defect after three days, he will be required to produce evidence that he (the slave) had the defect when he bought it." Abu Dawud said: This explanation is from the words of Qatadah.