Praise be to Allah.
We need to describe the beliefs (‘aqeedah) of the Shi’a so as to know the ruling on praying behind them.
For details on their beliefs, please see the answer to Question no. 4569.
With regard to praying behind one who is of this nature, Shaykh Ibn Baaz (may Allaah have mercy on him) said:
“It is not permissible to pray behind any of the mushrikeen, including those who seek the help of anyone other than Allaah and seek support from him, because seeking help through anyone other then Allaah, such as the dead, idols, the jinn, etc. is shirk, the association of others with Allaah, glorified be He.”
Fataawa al-Shaykh Ibn Baaz (vol. 2, p. 396)
And he said: one should not pray behind any imaam who is known to exaggerate concerning the Ahl al-Bayt (members of the Prophet’s household). If no such thing is known about him or any other Muslim, then it is OK to pray behind them.
Fataawa al-Shaykh Ibn Baaz, vol. 12, p. 107