Musnad Ahmad 28:326

Yahya ibn Sa'id narrated to us, from Abu Malik—meaning 'Ubayd ibn al-Akhnas—, al-Walid ibn 'Abd Allah narrated to me, from Yusuf ibn Mahak, from 'Abd Allah ibn 'Amr, who said: I used to write down everything I heard from the Messenger of Allah, may Allah bless him and grant him peace, wanting to memorize it. But Quraysh forbade me from that, and they said: "Do you write down everything, while the Messenger of Allah, may Allah bless him and grant him peace, speaks in anger and in pleasure?" So I refrained, until I mentioned that to the Messenger of Allah, may Allah bless him and grant him peace. He said: "Write, for by the One in whose hand is my soul, nothing comes out of it except the truth."