Abu Dawud Book 28, Hadith Number 4154.

Narated By ‘Aisha, Ummul Mu’minin : A woman made a sign from behind a curtain to indicate that she had a letter for the Apostle of Allah (PBUH). The Prophet (PBUH) closed his hand, saying: I do not know this is a man’s or a woman’s hand. She said: No, a woman. He said: If you were a woman, you would make a difference to your nails, meaning with henna.