Musnad Ahmad 30:1649

Hussain ibn Muhammad narrated to us, Sharik narrated to us, from Layth, from Ka'b, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Send blessings upon me, for it is a purification for you. And ask Allah to grant me the *Wasilah*, for it is a rank in the highest part of Paradise, which only one man shall attain, and I hope that I am he."