Is it a sunnah or a fard for women to put henna on their hands?

Hazrath Ayesha (Radhiallaahu Anha) narrates that Hinda, the daughter of Utbah said, Oh Prophet of Allah, grant me Bai'at (allegiance). Rasulullah (Sallallaahu Alayhi Wasallam) replied, I will not grant you Bai at until you change your palms (apply mehndi) - (Mishkat pg.383 -Rahimiya)

Hazrat Ayesha (Radhiallaahu Anha) states that a woman behind a curtain presented a letter to Rasulullah (Rasulullah (Sallallaahu Alayhi Wasallam). Rasulullah (Sallallaahu Alayhi Wasallam) withdrew his hand saying I do not know if this is a males hand or females hand, the woman replied, Rasulullah ((Sallallaahu Alayhi Wasallam) said, 'If you were a female you would have changed your nails, (by applying mehndi)' - (Ibid)

In view of the above quoted hadith one of the reasons for females applying mehndi is to distinguish her hands from that of a male. That can be best achieved by applying mehndi on the palms and the nails. That can also be achieved by applying mehndi on only the palms or only on the nails.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai