Musannaf Ibn Abi Shayba 5:760

Muhammad ibn Bakr narrated to us, from Ibn Jurayj, from Abu al-Zubayr, from Jabir, that he heard him say: Ibn Rawahah appraised it—meaning Khaybar—at forty thousand wasqs. He claimed that when Ibn Rawahah gave the Jews the choice, they took the dates (the produce), and upon them was twenty thousand wasqs.