Volume 2, Book 13, Number 745:
Narrated Abu Hummaid As-Sa’idi
One night Allah’s Apostle (p.b.u.h) stood up after the prayer and recited “Tashah-hud” and then praised Allah as He deserved and said, “Amma ba’du.”
Volume 2, Book 13, Number 746:
Narrated Al-Miswar bin Makhrama:
Once Allah’s Apostle got up for delivering the Khutba and I heard him after “Tashah-hud” saying “Amma ba’du.”
“Amma Ba`du” (أما بعد) literally means “as for after”. Khateebs and lecturers usually say it in their talks after praising Allah and before starting their own speech. It’s like separating the two things – what Allah and His Messenger (SAW) said, and what I have to say.
To get a clearer picture, go through this.