Musnad Ahmad 175:17

Uthman ibn Umar narrated to us, he said: Yunus informed us, from al-Zuhri, from Abd al-Rahman ibn Ka'b, from his father, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "The soul of the Muslim 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."