Musannaf Ibn Abi Shayba 13:1202

Abu Bakr narrated to us, he said: Ibn Abi Za’idah narrated to us, from ‘Abd al-Malik, from ‘Ata’, regarding a man who had dirhams owed to him by another man. When they became due, he said, "Keep them as a *mudarabah* (profit-sharing investment)." He said: "It is not valid until he takes possession of them from him, then he hands them over to him if he wishes."