Thanks, I figured out it also work without adding the prefix and added the E option to hang up the channel once the eavesdropped call has ended.
exten => 1,1,ChanSpy(,Ebqe(SIP/10))
Related solutions
http://forums.asterisk.org/viewtopic.php?f=1&t=78922
https://www.elastix.org/community/threads/chanspy-listening-in-to-all-but-these-extensions.119917/
https://www.elastix.org/community/threads/chanspy-to-specific-extension.25721/
Related threads
For reference, I am going to post all topics I found about this problem:
https://blogs.asterisk.org/2016/02/10/converting-from-chan_agent-to-app_agent_pool/
http://the-asterisk-book.com/1.6/applikationen-chanspy.html
https://wiki.asterisk.org/wiki/display/AST/Application_ChanSpy
http://forums.asterisk.org/viewtopic.php?f=1&t=96037