Musnad Ahmad 28:480

Rawh narrated to us, Shu'bah narrated to us, Husayn informed me, I heard Mujahid narrating from Abdullah ibn Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "For every action there is an intensity, and for every intensity there is a slackening. So whoever’s slackening is towards my Sunnah, then he has succeeded, and whoever’s is towards other than that, then he has perished."