What to Do if You Are Praying Sunnahs and the Iqaamah for the Fard Prayer is Called
by The Albaani Site
Questioner: Regarding the prayer in the second rak’ah, if, ya’ni, for example, you’re sitting, or you stood up from rukoo’ in the second rak’ah of [your] Sunnah prayers, and the iqaamah for the [fard] prayer has been said, should you cut off your prayer or complete it, [a prayer of which you are in] the second rak’ah after rukoo’ or you just stood up from rukoo’?
Al-Albaani: … if you think that you will be able to catch the opening takbeer [of the fard prayer] with the Imaam [even] if you complete your prayer, then you complete it, and if not then you break it.
Al-Hudaa wan-Noor, 529.
Also refer to this post.