python 3 simple solution

def closestNumbers(arr):
        arr.sort()
        _min = min([arr[i+1]-arr[i] for i in range(len(arr)-1)])
        pairs = [(arr[i], arr[i+1]) for i in range(len(arr)-1) if arr[i+1]-arr[i] == _min ]
        return reduce(lambda a, b: a+b, pairs)