Musannaf Ibn Abi Shayba 22:185

Abu Usama narrated to us, from ‘Amr ibn Hamza, from Salim, from Ibn ‘Umar, who said: ‘Umar asked me, “How much of the Qur’an do you have?” I said, “Ten surahs.” He then said to ‘Ubayd Allah ibn ‘Umar, “How much of the Qur’an do you have?” He said, “A surah.” ‘Abd Allah said: He did not forbid us, nor did he command us, other than that he said, “And if you are to learn anything from it, then upon you is this Mufassal, for it is preserved.”