Musannaf Ibn Abi Shayba 32:17

Muhammad ibn Fudayl narrated to us, from Abu Sinan, from Ya'qub ibn Ghadban al-Yashkuri, from Abdullah ibn Mas'ud, who said: A man came to him who had committed a sin and asked him about it. He (Ibn Mas'ud) ignored him and turned to the people with their conversation. Then a glance from Abdullah happened to fall upon the man, and behold, the man’s eyes were shedding tears. He said: "As for this, indeed, what you came to ask me about has concerned me. Verily, for Paradise there are seven gates, all of which open and close, except for the gate of repentance; an angel is appointed over it. So act, and do not despair."