Hadith: The happiest of people to receive my intercession on the Day of Judgment is the one who says, 'there is no god but Allah' sincerely from his heart or his soul

عَنْ أَبِي هُرَيْرَةَ رضي الله عنه أَنَّهُ قَالَ:
قِيلَ: يَا رَسُولَ اللهِ، مَنْ أَسْعَدُ النَّاسِ بِشَفَاعَتِكَ يَوْمَ الْقِيَامَةِ؟ قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ: «لَقَدْ ظَنَنْتُ يَا أَبَا هُرَيْرَةَ أَنْ لَا يَسْأَلَنِي عَنْ هَذَا الْحَدِيثِ أَحَدٌ أَوَّلُ مِنْكَ؛ لِمَا رَأَيْتُ مِنْ حِرْصِكَ عَلَى الْحَدِيثِ، أَسْعَدُ النَّاسِ بِشَفَاعَتِي يَوْمَ الْقِيَامَةِ، مَنْ قَالَ لَا إِلَهَ إِلَّا اللهُ خَالِصًا مِنْ قَلْبِهِ أَوْ نَفْسِهِ»».
[صحيح] - [رواه البخاري] - [صحيح البخاري: 99]

Abu Hurayrah (may Allah be pleased with him) reported:
It was said: "O Messenger of Allah, who will be the happiest of people to receive your intercession on the Day of Judgment?" The Messenger of Allah (may Allah's peace and blessings be upon him) said: "I thought no one would ask me about this Hadīth before you, O Abu Hurayrah, given the keenness I have seen from you regarding the Hadīths. The happiest of people to receive my intercession on the Day of Judgment is the one who says, 'there is no god but Allah' sincerely from his heart or his soul."
[Authentic hadith] - [Narrated by Bukhari] - [Sahih Bukhari - 99]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informs that the happiest of people to receive his intercession on the Day of Judgment is the one who says, "there is no god but Allah" sincerely from his heart; that's no one is worthy of worship except Allah, and he should be free from polytheism and show-off.