There is a hadith (graded hasan) in Abu Dawud that a women gave a book/letter to the Prophet from behind the veil. The Prophet saw her hand

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

This Hadeeth was reported by Imam Ahmed in his Musnad, and by Abu Daawood and an-Nasaa’i from ‘Aa’ishah who said: "A woman made a sign from behind a curtain to indicate that she had a letter for the Messenger of Allah . The Prophet closed his hand, saying: "I do not know whether it was the hand of a man or a woman. She said: 'It is the hand of a woman.' He said: 'If you were a woman, you would make a difference to your nails; meaning by dyeing them with henna.'"

This Hadeeth is classified as weak by a group of scholars. Imam Ahmad and Ibn al-Jawzi said: "It is an objectionable Hadeeth." Ibn al-Qattaan said: "It is very weak." Other scholars classified it as weak as well.

However, al-Albaani classified it as Hassan in some of his books, and as weak in some other books.

Some scholars provided evidence of this Hadeeth that a woman is permitted to show her hands. Ibn al-Qattaan mentioned this Hadeeth under (the issue of the two hands: what can be inferred as evidence for those who are of the view that it is permissible for her to show her hands) but he classified the Hadeeth as weak as we have already mentioned.

With regard to the view that both hands are an ‘Awrah and it is not permissible (for a woman) to show them: then it is possible that the woman in the Hadeeth is a woman of post-menstrual age, or that the Hadeeth was reported before the Hijaab was made obligatory.

In any case, we have already mentioned that the Hadeeth is weak, so there is no need to continue to elaborate on it.

Allah knows best.