Musannaf Ibn Abi Shayba 30:123

Yazid ibn Harun narrated to us, he said: Hammam narrated to us, he said: Zayd ibn Aslam narrated to us, from Ata ibn Yasar, from Ubadah ibn al-Samit, from the Prophet, may Allah bless him and grant him peace, who said: "Paradise has one hundred degrees. Between each degree is like what is between the heaven and the earth. Al-Firdaws is the highest of them in degree, and above it is the Throne, and from it gush forth the four rivers of Paradise. So when you ask Allah for Paradise, ask Him for al-Firdaws."