How develop Model and validation

Hi
In this my project(Shop),there are 4 model (User,Shop,Order,Category)
There is no problem
I want to know only,is it correct validate function argument in model or i have to validate it in controller?
Example
I have User model with Login function
This function have to arguments(username,password)
Do i have to check username and password in Model or in Controller validate and then use model?
Actually i want to know what is standard programming for use model and validation and split this from their