Hadith 31-276

وَبِإِسْنادِهِ عَنْ عَلِيِ‏ عَلَيْهِ السَّلامُ قالَ: لَقَدْ عِلْمَ المُستَحفِظُونَ مِنْ أَصْحابِ مُحَمَّدٍ أَن أَهْلَ صِفِّينَ قَدْ لَعَنَهُمُ اللَّه عَلَى لِسانِ نَبِيِّهِ، وَقَدْ خابَ مَنِ افْتَرى‏.

According to the same documentation, on the authority of Ali (a.s.), “The loyal companions of Muhammad remember that God cursed the people of Saffayn by means of the tongue of His Prophet. It does not matter if anyone lies about this.”