sinon stub is not working

I'm quiet new into testing and I don't seem to succeed to succesfully stub a function. I'm trying to stub the connection to the database, but it keep's contacting it, instead of using the result from the stub:

Here's the function:

var self = module.exports = {
VerifyAuthentication: function (data){      
    var deferred = q.defer()                
    if(typeof(data.email)=='undefined'){
        deferred.reject({data:{},errorcode:"",errormessage:"param 'email' is mandatory in input object"})
    }else{
        if(typeof(data.password)=='undefined'){
            deferred.reject({data:{},errorcode:"",errormessage:"param 'password' is mandatory in input object"})
        }else{
            var SqlString = "select id, mail, password, origin from tbl_user where mail = ?"    
            var param = [data.email]
            self.ExecuteSingleQuery(SqlString, param).then(function(results){   
                if(results.length > 0)
                {
                    if (results[0].password == data.password)
                    {                   
                        deferred.resolve({data:{"sessionId":results[0].id},errorcode:"",errormessage:""})
                    }else{              
                        deferred.reject({data:{},errorcode:"",errormessage:"bad password"})
                    }
                }else{
                    deferred.reject({data:{},errorcode:"",errormessage:"unknown user"})
                }
            })
        }
    }
    return deferred.promise
},
ExecuteSingleQuery: function (queryString, parameters){     
    var deferred = q.defer()        
    var connection = connect()

    connection.query(queryString, parameters, function (error, results, fields){
        if(error){ deferred.reject(error)};     
        deferred.resolve(results)
    });

    return deferred.promise
},

And here's the test:

var dbconnection = require('../lib/dbConnection.js') 
describe("VerifyAuthentication", function(){
    it("_Returns DbResult object when user name and password match", function(){
        var expectedResult = {data:{"sessionKey":"b12ac0a5-967e-40f3-8c4d-aac0f98328b2"},errorcode:"",errormessage:""}
        stub = sinon.stub(dbconnection, 'ExecuteSingleQuery').returns(Promise.resolve(expectedResult))
        return dbconnection.VerifyAuthentication({email:"correct@adres.com",password:"gtffr"}).then((result)=>{
            expect(result.data.sessionId).to.not.be.undefined
            expect(result.errorcode).to.not.be.undefined
            expect(result.errormessage).to.not.be.undefined
            stub.restore()                
        })
    })  
})

I always got an error 'unknown user', which is normal, because the user is indeed not in the database. However, I want to stub the 'ExecuteSingleQuery' function, avoiding it to connect to DB.