Musnad Ahmad 33:1131

Sulayman ibn Dawud narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from Musa ibn Uqbah, from Abu al-Zubayr, from Jabir, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, forbidding that a man should touch another man while under a single garment, and that a woman should touch another woman while under a single garment.