He who disapproves is guiltless, and he who disapproves is safe. Qatadah said : it means one who feels its disapproval in his heart, and one who expresses disapproval in his heart.
Sunan Abi Dawud Book 42, Hadith 166
← Previous Hadith The Messenger of Allah (May peace be upon him) said: You will have commanders some of whom you will approve and some of whom you will disapprove. He who expresses disapproval with his tongue (Abu Dawud said : This is Hisham’s version) is guiltless; and he who feels disapproval in his heart, is safe, but he who is pleased and follows them. He was asked; shall we not kill them, Messenger of Allah? Abu Dawud’s version has : Shall we not fight with them? He replied : No, so long as they pray.

Sunan Abi Dawud Book 42, Hadith 165 Next Hadith → various corruptions will arise in my community, so strike with sword the one who tries to cause separation in the matter of Muslims when they are united, whoever he be.

Sunan Abi Dawud Book 42, Hadith 167