Sunan an-Nasa'i 3938
It was narrated from Az-Zuhri concerning two slaves who were partners, and one of them quit, that he said:
"One of them may cover for the other if they were partners."
أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ أَنْبَأَنَا ابْنُ الْمُبَارَكِ، عَنْ يُونُسَ، عَنِ الزُّهْرِيِّ، فِي عَبْدَيْنِ مُتَفَاوِضَيْنِ كَاتَبَ أَحَدُهُمَا قَالَ جَائِزٌ إِذَا كَانَا مُتَفَاوِضَيْنِ يَقْضِي أَحَدُهُمَا عَنِ الآخَرِ‏.‏‏