Ref. No. 1206/42-555
In the name of Allah the most Gracious the most merciful
The answer to your question is follows:
The Hadith e Kisaa is reported in authentic books. The Hadith is:
عَائِشَةُ قَالَتْ خَرَجَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ غَدَاةً وَعَلَيْهِ مِرْطٌ مُرَحَّلٌ ، مِنْ شَعْرٍ أَسْوَدَ ، فَجَاءَ الْحَسَنُ بْنُ عَلِيٍّ فَأَدْخَلَهُ ، ثُمَّ جَاءَ الْحُسَيْنُ فَدَخَلَ مَعَهُ ، ثُمَّ جَاءَتْ فَاطِمَةُ فَأَدْخَلَهَا ، ثُمَّ جَاءَ عَلِيٌّ فَأَدْخَلَهُ ، ثُمَّ قَالَ: {إِنَّمَا يُرِيدُ اللَّهُ لِيُذْهِبَ عَنْكُمُ الرِّجْسَ أَهْلَ الْبَيْتِ وَيُطَهِّرَكُمْ تَطْهِيرًا{رواہ مسلم مشکوۃ شریف 2/528 باب مناقب اھل بیت النبی ﷺ وکذا فی الترمذی حدیث عمرو بن ابی سلمۃ 2/219 مناقب اھل البیت ، نعیمیہ دیوبند۔
As far as the statement of Hazrat Ali is concerned, though this sentence is not found in any book of Hadith, yet, it is mentioned in some historical books, but the Shiites do not have right to apply it to themselves. The time Hazrat Ali (ra) spoke that sentence, there were no Shiites. Then his sentence means that by Allah we have succeeded and our Jamaat has succeeded. Shia means Jamaat. So the existing Shia sect is not meant in the sentence.
And Allah knows best
Darul Ifta
Darul Uloom Waqf Deoband