Musnad Ahmad 941:82

Muhammad ibn Abdullah ibn al-Muthanna narrated to us, Salih ibn Abi al-Akhdar narrated to me, al-Zuhri narrated to me, from Urwah, from Usamah, that the Prophet, peace and blessings of Allah be upon him, had intended an expedition. Then the Prophet, peace and blessings of Allah be upon him, passed away. Abu Bakr, may Allah be pleased with him, asked him [Usamah]: "What did he entrust to you?" He said: "He entrusted me to raid Ubna in the morning, and then to burn."