+1(514) 937-9445 or Toll-free (Canada & US) +1 (888) 947-9445
You can't get it back but you can get a verification of having landed. Apply here: http://www.cic.gc.ca/english/information/applications/certcopy.asp
 
I did send my passport to immigration office for multiple entry with landing paper . But I revived my passport without landing paper .
 
You can contact the office where you sent the passport to and ask them if they have it. However, you should not have left your landing papers in your passport while sending it somewhere for a visa. These are important papers for yourself to keep.