Musannaf Ibn Abi Shayba 21:470

Ubayd Allah ibn Musa narrated to us, from Musa ibn Ubaydah, from Muhammad ibn Amr ibn Ata, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Ask Allah for the Wasilah for me. No believer asks for it for me in the world, except that I will be a witness for him or an intercessor on the Day of Resurrection."