Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “The most blessed of people to have my intercession on the Day of Resurrection are those who say there is no God but Allah, sincerely from the heart.”
Source: Ṣaḥīḥ al-Bukhārī 99
Grade: Sahih (authentic) according to Al-Bukhari
عَنْ أَبِي هُرَيْرَةَ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَسعَدُ النَّاسِ بِشَفَاعَتِي يَوْمَ الْقِيَامَةِ مَنْ قَالَ لَا إِلَهَ إِلَّا اللَّهُ خَالِصًا مِنْ قَلْبِهِ
99 صحيح البخاري كتاب العلم باب الحرص على الحديث