Musnad Abu Ya'la al-Mawsili 160:19

Al-Hasan ibn 'Arafah narrated to us, 'Ammar ibn Muhammad narrated to us, from Layth ibn Abi Sulaym, from Ka'b, from Abu Hurayrah, who said: The Messenger of Allah (ﷺ) said: "Increase your prayers upon me, for your prayers upon me are a purification for you. And ask for the *Wasilah* for me." It was said: "O Messenger of Allah, what is the *Wasilah*?" He said: "The highest degree in Paradise. No one shall attain it except one man among the people, and I hope that I am he."