Musannaf Ibn Abi Shayba 27:146

Ibn Fudayl narrated to us, from Layth, from Ka'b, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Send blessings upon me, for sending blessings upon me is a purification for you. Ask Allah for the *Wasilah* for me." They said: "And what is the *Wasilah*, O Messenger of Allah?" He said: "The highest degree in Paradise, which only one man will attain, and I hope that I am he."