Musnad Ahmad 30:583

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Muhammad ibn Wasi', from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever relieves a distress for a distressed person in the world, Allah will relieve a distress for him in the Hereafter. And whoever conceals the nakedness (faults) of a Muslim in the world, Allah will conceal his nakedness (faults) in the Hereafter. And Allah is in the aid of the servant as long as he is in the aid of his brother."