Musannaf Ibn Abi Shayba 31:39

Muhammad ibn Fudayl narrated to us, from al-A’mash, from Abu Salih, from Abu Hurayrah, who said: Ibn Mas’ud said to Abu Hurayrah, "Do you see how thick the skin of the disbeliever is?" Abu Hurayrah said, "I do not know." Then ‘Abdullah said, "The thickness of the skin of the disbeliever is forty-two cubits."