It was Iron Fist Tan and his brother. They did it.
7 seconds sound clip from the Kung Fu - The Punch of Death movie soundboard.
You can hear this line at 00:28:35 in the DVD version of the movie.
Quote context
[...]
- Can't tell you, that's right, sir, can't tell.
- Come on. Tell me who killed him.
- In that case, I'll have to assume that you did it.
- Tell me.
- Yeah, yeah, yeah, all right.
- Tell me.
- It was Iron Fist Tan and his brother. They did it.
- What's wrong? You've told the boy?
- Oh, we didn't have any choice, though... said he'd kill us. So then we had to tell him.
- Quickly... see if he's already gone. Hurry.
- Yes.
(SOUNDS OF COMBAT PRACTICE) (5)
[...]