Question 270: assalam alaykum. when a shia is in a sunni jamaat prayer And lifts his hands for Qonoot in the second rakaat as they go for rokoo is it still considered as a jamaat prayer?
Answer 270: If you know that if you complete qunut, you will not be able to join the Imam in his Ruku, yet you purposely recite qunut, and miss the Imam in Ruku, your congregational prayer will be void, and should act accordingly to the rules of Furada prayers.
Otherwise, if you are sure that if you recite qunut you will not miss the Imam in Ruku, but when you recite qunut and miss the Imam in Ruku, your congregational prayer is in order.
As you know it is Mustahab that qunut be recited in all obligatory and Mustahab prayers before the Ruku of the second Rak’at. So, if you don’t recite it there would be no problem in it and your prayers would be in order.
For further information in this regards, please refer to the following answer:
Index: Number of things which invalidate the prayer, answer 547.
Index: Qualification of an Imam of congregational prayers, answer 021.
Index: The differences and similarities between Shia and Sunni, answer 187.
Index: Offering prayer behind a Sunni Imam, answer 237.
 . The official website of the office of Sayyid Sistani (ha), obligatory acts relating to Namaz, issues 1452&1453; Tawzih al-Masael of maraja’, Vol. 1, issues 1117-1121.