Musnad Ahmad 30:2108

Ahmad ibn Abd al-Malik ibn Waqid al-Harrani narrated to us, he said: Muhammad ibn Salamah narrated to me, from Muhammad ibn Ishaq, from Amr ibn Shu’ayb, from Mujahid and al-Mughirah ibn Hakim, from Abu Hurayrah, they both said: We heard him saying: "There was no one more knowledgeable of the hadith of the Messenger of Allah, may Allah bless him and grant him peace, than me, except for what was with Abdullah ibn Amr; for he used to write with his hand and memorize it in his heart, while I used to memorize it in my heart and not write with my hand. He sought permission from the Messenger of Allah, may Allah bless him and grant him peace, to write from him, and he permitted him."