Answered by Shaykh Faraz Rabbani
Recently Shaykh Nuh was asked the perennial question, if it was alright to pray behind a Wahabi (!). His answer was that he does, and does not repeat the prayer. I was confused.

Walaikum assalam wa rahmatullah,
It is superior to pray behind an innovator (or corrupt person) than to pray alone. [Ibn Abidin, Radd al-Muhtar]
This is also the way of my teachers in Damascus, and the fatwa of Habib Zayn ibn Sumayt, the top Yemeni Shafii faqih–based in Madina, who quoted Imam Anwar Shah al-Kashmiri in support of his answer. It is also what major Indian fuqaha say in their works and answers–such as Mufti Mahmoud Ashraf Usmani.
Shaykh Muhammad Ba-Dhib, a notable young Yemeni scholar from a very noble family of scholars and saints from Tarim, was here in Amman recently. He expressed his amazement that some people will not pray in congregation in the Haramayn and elsewhere because the imam was Wahhabi.
The basis of any individual’s prayer is that it is valid and sound. This is our operational assumption about all human acts. To hold otherwise–without proof reaching reasonably surety–is to hold ill-judgment of others without basis.
Thus, we assume their prayer is sound, and pray. This is what the texts say, and this is what the fuqaha tell us.
As long as the imam does not do anything that certainly invalidates their prayer according to all of Sunni scholarship, one prays behind them.
At the same time, every Muslim serious about their religion should be aware of the way of Sunni Islam in beliefs, actions, and spirituality, and the divergence of the Salafi/Wahhabi movement in each of these 3 areas from sound scholarship.
The articles by Shaykh Nuh Keller, Shaykh Abdal Hakim Murad and others at Sidi Mas’ud Khan’s excellent site www.masud.co.uk are essential in this regard.
Wassalam.
Faraz Rabbani.