It is narrated from Hazrat Aisha (may Allah be pleased with her) that once a woman, from behind a veil, extended her hand and presented a letter to the Prophet (peace be upon him). The Prophet (peace be upon him) withdrew his hand and said, "How do I know whether this is the hand of a man or a woman?" The woman replied, "I am a woman." The Prophet (peace be upon him) said, "If you are a woman, then color your nails with henna."