سنن النسائي

It was narrated that Muhammad said: "I do not know that Shuraih ever ruled on Mudarabah disputes except in two ways. He would say to the Mudarib (the one who contributed his labor to the partnership): 'You must provide proof that a calamity befell you so that you may be excused.' Or he would say to the one who invested his money in the partnership: 'You must provide proof that your trustee betrayed his trust, otherwise his oath sworn by Allah that he did not betray you is sufficient.'"