Musnad Abu Ya'la al-Mawsili 155:56

Abu Bakr ibn Abi al-Nadr narrated to us, he said: Abu al-Nadr narrated to me, he said: Ibrahim ibn Sa'd narrated to us, from his father, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "People will enter Paradise whose hearts are like the hearts of birds."