Musnad Ahmad 1133:1

Muhammad ibn Humayd Abu Sufyan narrated to us, from Ma'mar, from al-Zuhri, from 'Abd al-Rahman ibn 'Abd Allah ibn Ka'b, from Ka'b, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The soul of the believer hangs in the trees of Paradise, until Allah returns it to his body."