Hadith 31-306

وَبِإِسْنادِهِ عَن الحُسَيْنِ بْنِ عَلِى‏ عَلَيْهِ السَّلامُ قالَ: ما كُنَّا نَعرِفُ المُنافِقِينَ عَلَى عَهْدِ رَسُولَ اللَّهِ‏ صَلَّى اللهُ عَلَيْهِ وَآلِهِ إِلاّ بِبُغضِهِم عَلِيّاً وَوُلْدِه‏ عَلَيْهِمُ السَّلاَمُ.

According to the same documentation, on the authority of Al-Husayn ibn Ali (a.s.), “During the time of God’s Prophet (S) we could not recognize the hypocrites except by their animosity towards Ali (a.s.) and his progeny.”