Musnad Ahmad 32:599

'Abd al-Samad narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The people of Paradise will enter Paradise, and there will remain of it what Allah wills. Then Allah will create for it—meaning for Paradise—a creation, until He fills it."