Why is my mobile phone and charger is heating so much while charging my phone ?

I have purchased Xiaomi Redmi Note 3 (32 gb , gold) variant last week.
But when I am charging my phone , both the mobile phone and the charger
is heating too much and when the battery status becomes 95 % , it starts cooling
down slowly and becomes warm.
Is there any problem in the phone or the charger ?
Should I replace my phone ?
Suggest me some tips and solutions.

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