Musnad Abu Ya'la al-Mawsili 42:302

Zuhayr narrated to us, al-Hasan ibn Musa narrated to us, Ibn Lahi'ah informed us, from Darraj, from Abu al-Haytham, from Abu Sa'id, from the Prophet (peace and blessings of Allah be upon him), that he said: "Indeed, the distance between the two leaves of a door in Paradise is a journey of forty years."