Musnad Ahmad 26:625

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, from Abu Wa'il, from Abdullah, from the Prophet, may Allah bless him and grant him peace, that he said: "If you are three, then two of you must not whisper privately to the exclusion of their companion, for that would sadden him. And a woman must not touch another woman, for that would cause her to describe her to her husband."