How long does a retired Military person recieve retirement pay? Where can I find writen proof?

I was asked By a Mortgage company to prove that in 3 years from know I would continue to receive a military retirement check. I know its written some place but on mil.com va.gov etc. I can't find the answer.
Update: Just a quick note mortgage company is WellsFargo and I have given them 2 copies of my 1099 and 2 DFAS-CL 7220/148 Retiree account statements (annual new pay statements)
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