Musnad Ahmad 28:65

Yazid narrated to us, Hariz informed us, Hibban al-Shar'abi narrated to us, from Abdullah ibn Amr ibn al-As, from the Prophet, may God bless him and grant him peace, that he said while he was on the pulpit: "Show mercy, and you will be shown mercy; and forgive, and God will forgive you. Woe to the vessels of speech. Woe to those who persist, who persist in what they have done while they know."