Musnad Ahmad 31:782

Musa ibn Dawud narrated to us, from Ibn Lahi'ah, from Musa ibn Wardan, who said: I heard Abu Sa'id al-Khudri saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Wasilah is a rank with Allah, above which there is no rank. So ask Allah to grant me the Wasilah."