Majlesee in “Bihar al anwar”:

“From a free-slave of Alee ibn Husayn [ibn Alee may Allaah be pleased with them] that he said : ‘I was with him (i.e. Alee ibn Husayn may Allaah be pleased with him) in one of the situations where he was alone, so I said : ‘Indeed I have a right upon you , that you inform me about these two men : Aboo Bakr and Umar?’

So he replied : “Kaafiraan [They are two kaafirs /disbelievers] , Kaafir man ahabbahumaa [And whoever loves them is a kaafir].”