Musnad Ahmad 33:547

Husayn ibn Muhammad narrated to us, Sulayman ibn Qarm narrated to us, from Abu Yahya al-Qattat, from Mujahid, from Jabir ibn Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The key to Paradise is prayer, and the key to prayer is purification." Abdullah ibn Ahmad said: This is how it appeared in the original manuscript as "Hasan," but the correct [name] is "Husayn."