Sunan Abi Dawud 4687
سنن أبي داود#4687SahihIbn ‘Umar reported the Messenger of Allah (May peace be upon him) as saying :If any believing man calls another believing man an unbeliever, if he is actually an infidel, it is all right ; if not, he will become an infidel
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 42:92