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where are you applying from, what does your visa office website say and what does your appendix A say?
 
Actually i asked to Rupeshhari about ' Explanation Letter ' as model for myself, as i need to prepare a same latter too but have no idea how to, surely his English is better then me.

You need letter or not, above statement that i pasted is visa specific only, you cant find that at CIC website. This is what i got from my visa office which they are requesting me to proof of every deposites and even proof of sources where from i got that money too. Its maybe particular situation follow to my personal profile, as i know many applicants from same visa office and they never got this kind of requirements.

So please follow guide and document check list of your visa office. If you visa office require above all, then you have to do same as i'll, i'll let you know. You will have a better idea , what you need to do once you read creafully all requirements and informations on your visa specific guide. good luck
 
i think one statement or bank balance should be okay for buffalo. maybe someone else will correct me if i am wrong.
 
i agree with rupeshshari, nowehere in the documentation do they mention anything about retrospective bank statements. i think this is just unnecessary fluff, i feel they are more concerned with your credentials, experience and language skills initially. you will still have to prove funds when you land...in any case who says that the fact that you had a consistent balance in your account for the past 6 months means that those funds will be available a year from now when your app is going to be approved.