Musnad Ahmad 175:15

Muhammad ibn Idris—meaning al-Shafi‘i—narrated to us, from Malik, from Ibn Shihab, from ‘Abd al-Rahman ibn Ka‘b ibn Malik, that he informed him that his father, Ka‘b ibn Malik, used to narrate: That the Messenger of Allah (may Allah bless him and grant him peace) said: "The soul of the believer is but a bird that feeds upon the trees of Paradise, until Allah, Blessed and Exalted is He, returns it to its body on the day He resurrects it."