Musnad Ahmad 483:0

It was narrated from Habib ibn Maslamah al-Fihri that he said: I heard the Messenger of Allah (ﷺ) say: "The souls of the martyrs are in the crops of green birds, having lamps hanging from the Throne. They roam freely in Paradise wherever they wish, then they return to those lamps. Their Lord looked down upon them and said: 'Do you desire anything?' They said: 'What more could we desire, when we roam freely in Paradise wherever we wish?' He asked them this three times. When they saw that they would not be left without being asked, they said: 'O Lord, we want You to return our souls to our bodies so that we may be killed in Your cause once again.' When He saw that they had no need, they were left alone."