A woman has to cover her body when praying even when she is behind her husband
Question

Does my wife have to wear hijab in prayer when it is just her and I in prayer alone
Praise be to Allah.When a woman is praying, even if she is with her husband, her situation is different than at other times. Covering the ‘awrah is one of the conditions of the prayer being valid, for both men and women. Her prayer cannot be valid otherwise. It was narrated in a saheeh report from ‘Aa’ishah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) said: “Allah does not accept the prayer of a woman of child-bearing age unless (she covers herself) with a khimaar.” (Narrated by Abu Dawood, al-Salaah, 546; classed as saheeh by al-Albaani in Saheeh Sunan Abi Dawood, 596). What this means is that the prayer of an adult woman is not valid (if she does not cover herself), because if something is not accepted, it is not valid. 
Shaykh al-Islam [Ibn Taymiyah] said: If a woman is praying alone, she is commanded to cover herself… so the person who is praying covers during the prayer things which he or she is allowed to leave uncovered outside of prayer.
(See Majmoo’ al-Fataawa, vol. 22, p. 109) 
On this basis, a woman is required to cover all of her body during prayer apart from the face and hands. See question no. 1046 concerning the woman’s ‘awrah  which she has to cover during prayer. 
And Allah knows best.