Musnad Ahmad 33:719

Ibrahim ibn Abi al-Abbas narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Musa ibn Uqbah, from Abu al-Zubayr, from Jabir ibn Abdullah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "A man shall not touch a man skin-to-skin under one garment, and a woman shall not touch a woman skin-to-skin under one garment."