Musnad Ahmad 26:147

Waki' narrated to us, al-Mas'udi narrated to us, from Simak, from 'Abd al-Rahman ibn 'Abd Allah ibn Mas'ud, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) gathered us while we were forty. 'Abd Allah said: I was among the last of those who came to him. He said: "You will be afflicted, and you will be granted victory, and [lands] will be opened for you. So whoever among you lives to see that, let him fear Allah, and let him enjoin the good, and forbid the evil. And whoever lies about me intentionally, let him take his seat in the Fire."