Shafaqna (International Shia News Agency) – The Grand Ayatollah Sistani answered some questions regarding prayer behind a Sunni prayer leader. According to Shafaqna the questions and answers are as follows:
Question: Is it permissible to offer congregational prayers behind a Sunni prayer leader?
Answer: It is permissible but you must recite Hamd and Sura yourself even though the recitation may be in low voice.
Source: Ayatollah Sistani