What is the Mediation (Waasitah) of the Messenger?

The mediation of the Messenger is the transmission of Allah’s Message.

Evidence from the Noble Qur'an
{O Messenger (Muhammad) proclaim what has been sent down to you from your Lord}, {Soorah al-Maa.idah, verse 67]

Evidence from the authentic Sunnah
(('O Allaah, have I proclaimed your message? 'O Allaah bear witness. (In response to the statement of the Sahaabah (Companion), "Indeed, we bear witness that you have proclaimed the message."))), [Transmitted by Muslim]