Engaging in Sunnah prayers during the time of Jama'at
When the congregation (Jama'at) has stood up, it is not correct to engage in Sunnah prayers, because the Messenger of Allah ﷺ said: "When the congregation is established, there is no prayer except the obligatory (Fard) prayer." Yes, if there is no fear of missing the Fajr prayer while performing the Sunnah of Fajr, meaning one can catch at least one Rak'ah with the Imam, or can join the Imam in the Qa'dah (sitting posture), then one should perform the Sunnah of Fajr. (Aham Masail / Vol: 1 / Pg: 33)