I don't think sar provides that info.
Its a little difficult to post-analyse stuff like that if you weren't already setup to record the relevant info.
However, if you search through the various logfiles (/var/log/...), you should be able to see what was running in that hour.
That will give you a start.
There are various monitoring tools available eg
http://collectl.sourceforge.net/
A quick way of finding out next time, is to setup top to run (in advance) in batch mode
http://linux.die.net/man/1/top between 6-7am.
How can I find out the process which caused this ? I'm new here 
Please help me out. Thank you
