The Prophet (ﷺ) said: The contractual obligation of a slave is three days.
Sunan Abi Dawud Book 24, Hadith 91
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I sold it, that is, camel, to the Prophet (ﷺ), but I made the stipulation that I should be allowed to ride it to home. At the end he (the Prophet) said: Do you think that I made this transaction with you so that I take your camel ? Take your camel and its price; both are yours.
Sunan Abi Dawud Book 24, Hadith 90 Next Hadith → "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.
Sunan Abi Dawud Book 24, Hadith 92
Sunan Abi Dawud Book 24, Hadith 90 Next Hadith → "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.
Sunan Abi Dawud Book 24, Hadith 92