As far as I understand I have to use usrp internal clock or an external clock which I have to connect it on both usrp and oscilloscope?
It depends on what you are doing.
If you are doing a transmission yourself and you want to check the results, you will have a symbol clock you can use. It´s the clock that will mark the transmission of each symbol, as its name says.
If you are just receiving a signal someone else is transmitting, you need to derive that clock yourself. Doing a Google search for terms such as QAM QPSK I+Q clock reconstruction should yield some useful answers, there are several lab notebooks from universities available.