Musnad Ahmad 1012:30

Yazid narrated to us, he said: Hammam ibn Yahya narrated to us, Affan narrated to us, Hammam narrated to us, Zayd ibn Aslam narrated to us, from Ata ibn Yasar, from Ubadah ibn al-Samit, from the Prophet, peace and blessings of Allah be upon him, who said: "Paradise has one hundred levels; between every two levels is the distance of a one-hundred-year journey." And Affan said: "Like what is between the heaven and the earth. And al-Firdaws is the highest of them in level, and from it flow the four rivers, and the Throne is above it. So when you ask Allah, ask Him for al-Firdaws."