The Prophet ﷺ said: The contractual obligation of a slave is three days.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ عُهْدَةُ الرَّقِيقِ ثَلاَثَةُ أَيَّامٍ  

"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.  

وَمَعْنَاهُ زَادَ إِنْ وَجَدَ دَاءً فِي الثَّلاَثِ اللَّيَالِي رُدَّ بِغَيْرِ بَيِّنَةٍ وَإِنْ وَجَدَ دَاءً بَعْدَ الثَّلاَثِ كُلِّفَ الْبَيِّنَةَ أَنَّهُ اشْتَرَاهُ وَبِهِ هَذَا الدَّاءُ