Musnad Ahmad 1153:2239

Hassan ibn Musa narrated to us, he said: Muti’ ibn Maymun al-‘Anbari, whose kunya is Abu Sa’id, narrated to us, he said: Safiyyah bint ‘Ismah narrated to me, from ‘Aishah, the Mother of the Believers, she said: A woman extended her hand from behind a curtain with a letter to the Messenger of Allah, may Allah bless him and grant him peace. The Prophet, may Allah bless him and grant him peace, withdrew his hand and said: "I do not know if it is the hand of a man or the hand of a woman." She said: "Rather, it is a woman." He said: "If you were a woman, you would have changed your nails with henna."