Musannaf Ibn Abi Shayba 27:577

Narrated to us Abd al-Rahim, from Zakariyya, from Amir, who said: The Prophet, peace and blessings of Allah be upon him, set out, and al-Abbas was with him. Al-Abbas was a man of sound judgment. The Prophet, peace and blessings of Allah be upon him, said: "O uncle, if you see a mistake, then command me regarding it."