Sahih Bukhari Volume 1, Book 3, Hadith 99

حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنِي سُلَيْمَانُ، عَنْ عَمْرِو بْنِ أَبِي عَمْرٍو، عَنْ سَعِيدِ بْنِ أَبِي سَعِيدٍ الْمَقْبُرِيِّ، عَنْ أَبِي هُرَيْرَةَ، أَنَّهُ قَالَ قِيلَ يَا رَسُولَ اللَّهِ، مَنْ أَسْعَدُ النَّاسِ بِشَفَاعَتِكَ يَوْمَ الْقِيَامَةِ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ 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 قَلْبِهِ أَوْ نَفْسِهِ ‏"

Narrated By Abu Huraira : I said: "O Allah's Apostle! Who will be the luckiest person, who will gain your intercession on the Day of Resurrection?" Allah's Apostle said: O Abu Huraira! "I have thought that none will ask me about it 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 sincerely from the bottom of his heart "None has the right to be worshipped but Allah." And 'Umar bin 'Abdul 'Aziz wrote to Abu Bakr bin Hazm, "Look for the knowledge of Hadith and get it written, as I am afraid that religious knowledge will vanish and the religious learned men will pass away (die). Do not accept anything save the Hadiths of the Prophet. Circulate knowledge and teach the ignorant, for knowledge does not vanish except when it is kept secretly (to oneself)."