بسم الله الرحمن الرحيم

Question:
What is the sunnah for a woman praying with her husband? Does she line up next to him or behind him?

Answer:
Its all the same; there is no prohibition if she lines up beside him and there is no prohibition if she lines up behind him.

http://alfawzan.af.org.sa/node/14854
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N.B With regards to the above fatwa then it should be noted some scholars do view the issue to be somewhat open however the overwhelming opinion based upon hadith of Anas* is that the woman stands behind, this is the opinion of many of the scholars past and present that the woman prays behind the man regardless of whether it is her husband or other than him, from these scholars are Shaykhul Islaam ibn Taymiyyah, Shaykh Muqbil bin Haadi al Waadi’ee, Shaykh bin Baz and Shaykh Ubayd Al Jaabi’ree

*Al-Bukhaari (380) and Muslim (658) narrated from Anas ibn Maalik that his grandmother Mulaykah invited the Messenger of Allaah (صلى الله عليه و سلم) to come and eat some food that she had prepared for him. He ate some, then he said: “Get up and I will lead you in prayer.” Anas said: The Messenger of Allaah (صلى الله عليه و سلم) stood up, and the orphan and I stood in a row behind him, and the old woman stood behind us, and the Messenger of Allaah صلى الله عليه و سلم) led us in praying two rak’ahs, then he left.
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Shaykh bin Baz was informed by a man that he used to pray next to his wife in and then being informed that it is impermissible for them to do so, he asked do we have to repeat our prayers?

The shaykh replied there is nothing upon you and do not repeat them, however do not do it again, and the shaykh continued to mention the above narration of Anas -radiallaahu anhu- and finished by saying the woman prays behind the men even if she was praying with her husband.
(Paraphrased)

http://www.binbaz.org.sa/mat/20810

Shaykh Ubayd’s answer regarding this issue:

http://ar.miraath.net/fatwah/3038/open-meeting-obaid/05-04-1433h