In CSMA/CD ,in worst case maximum time to reach the frame bit to other end is Tp and then Tp time is required to get back the collision signal to node again.So it takes one RTT and at that time node still should be sending the data,then only it can be sure that the collision signal it received,belonged to its frame.And it have to retransmit the packet again.
So Transmission Time >= 2* Propogation Time
In this case sender is sending the data until collision signal is reached back to sender,so it can be make sure whether its packet got involved in collision or not (including worst case too).