Musannaf Ibn Abi Shayba 37:94

Abu Usama narrated to us, he said: Ibn Jurayj narrated to us, from Harun ibn Abi A’isha, from ‘Adi ibn ‘Adi, from Salman ibn Rabi’ah, from ‘Umar, who said: "There will be leaders and officials; accompanying them is a trial (fitnah), and separating from them is disbelief." He (the narrator) said: I said, "Allah is the Greatest! Repeat it to me, O Commander of the Faithful, for you have relieved me." So he repeated it to him. Salman ibn Rabi’ah said: Allah said, "And fitnah is worse than killing," and fitnah is more beloved to me than killing.