Musnad Ahmad 28:92

Abu Abd al-Rahman narrated to us, Haywah narrated to us, Ka'b ibn Alqamah informed us that he heard Abd al-Rahman ibn Jubayr saying that he heard Abdullah ibn Amr ibn al-As saying that he heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "When you hear a caller to prayer, say the like of what he says. Then, invoke blessings upon me, for whoever invokes one blessing upon me, Allah will bless him ten times for it. Then, ask for the *Wasilah* for me, for it is a station in Paradise that is not appropriate except for one servant among the servants of Allah, and I hope that I am he. So, whoever asks for the *Wasilah* for me, intercession will be granted to him."