Musnad Ahmad 28:34

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah ibn al-Akhnas, he said: Al-Walid ibn 'Abd Allah informed us, 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. Then Quraysh forbade me, saying: "You write down everything you hear from the Messenger of Allah, may Allah bless him and grant him peace, and the Messenger of Allah, may Allah bless him and grant him peace, is a human being who speaks in anger and in pleasure." So I refrained from writing. Then I mentioned that to the Messenger of Allah, may Allah bless him and grant him peace. He said: "Write, for by Him in whose hand is my soul, nothing comes out of me except the truth."