Narrated Abu Huraira:

I said, "O Allah's Apostle! Who will be the luckiest person who will gain your intercession on the Day of Resurrection?" The Prophet ﷺ said, "O Abu Huraira! I have thought that none will ask me about this Hadith before you, as I know your longing for the (learning of) Hadiths. The luckiest person who will have my intercession on the Day of Resurrection will be the one who said, 'None has the right to be worshipped but Allah,' sincerely from the bottom of his heart."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ جَعْفَرٍ، عَنْ عَمْرٍو، عَنْ سَعِيدِ بْنِ أَبِي سَعِيدٍ الْمَقْبُرِيِّ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ أَنَّهُ قَالَ قُلْتُ يَا رَسُولَ اللَّهِ مَنْ أَسْعَدُ النَّاسِ بِشَفَاعَتِكَ يَوْمَ الْقِيَامَةِ فَقَالَ
" لَقَدْ ظَنَنْتُ يَا أَبَا هُرَيْرَةَ أَنْ لاَ يَسْأَلَنِي عَنْ هَذَا الْحَدِيثِ أَحَدٌ أَوَّلُ مِنْكَ، لِمَا رَأَيْتُ مِنْ حِرْصِكَ عَلَى الْحَدِيثِ، أَسْعَدُ النَّاسِ بِشَفَاعَتِي يَوْمَ الْقِيَامَةِ مَنْ قَالَ لاَ إِلَهَ إِلاَّ اللَّهُ. خَالِصًا مِنْ قِبَلِ نَفْسِهِ ".