Sahih Muslim
The Book Pertaining to Paradise, Its Description, Its Bounties and Its Intimates (Kitab Al-Jannat wa Sifat Naimiha wa Ahliha)
Book Number: 040 / Hadith Number: 6808
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: There would enter Paradise people whose hearts would be like those of the hearts of birds.