#python 3 solution
from itertools import pairwise

def closestNumbers(arr):
    arr.sort()
    minDiff = max(arr)-min(arr)
    pairs = []
    #finds minimum difference
    for i,j in pairwise(arr):
        minDiff = min(minDiff,j-i)
    #adds pairs with minimum difference to output
    for i,j in pairwise(arr):
        if j-i == minDiff:
            pairs.extend([i,j])
    return pairs