Musannaf Ibn Abi Shayba 17:1126

Yahya ibn Sa'id narrated to us, from Ubayd Allah ibn al-Akhnas, from al-Walid ibn Abd Allah, 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), intending to memorize it. Then, the Quraysh forbade me from doing that. They said, "Do you write down everything you hear from the Messenger of Allah (may Allah bless him and grant him peace), while the Messenger of Allah (may Allah bless him and grant him peace) speaks in states of pleasure and anger?" So, I refrained. Then I mentioned that to the Prophet (may Allah bless him and grant him peace), and he pointed with his hand to his mouth and said, "Write, for by the One in whose hand is my soul, nothing comes out of it except the truth."