From Sahih Bukhari (Hadith 6570):
Narrated Abu Huraira:I said, "O Allah's Messenger (ﷺ)! 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
Reference: Bukhari 6570