Musnad Ahmad 2:96

Jarir narrated to us, from Abd al-Malik ibn Umayr, from Jabir ibn Samurah, who said: Umar addressed the people at al-Jabiyah and said: "The Messenger of Allah (peace and blessings of Allah be upon him) stood in a position like this position of mine and said: 'Treat my companions well, then those who follow them, then those who follow them. Then there will come a people, one of whom will swear an oath before he is asked to swear it, and he will bear witness before he is asked to bear witness. So whoever among you desires to attain the center of Paradise, let him hold fast to the community (al-jama'ah), for the devil is with the one, and he is further away from the two. And let no man be alone with a woman, for the third of them is the devil. And whoever among you is pleased by his good deed and saddened by his evil deed, then he is a believer.'"