Sunan Ibn Majah, Chapter: The Book of the Sunnah

"I used to visit Ibn Mas'ud every Thursday afternoon, but he never uttered the words: 'The Messenger of Allah (saw) said.' Then one evening, he said: 'The Messenger of Allah (saw) said,' then he let his head hang down." He said: "I looked at him and saw that his shirt was unfastened; his eyes were filled with tears, and his veins were bulging out (with fear). He said: 'Or more than that, or less than that, or something similar.'" (Sahih)