The World Federation One Stop Fiqh

Ruling 1509

If a follower joins [the Eid prayer] when the imam has already said some of the takbīrs, then after the imam goes into rukūʿ, he must say all the takbīrs and perform all the qunūts that he missed and then perform rukūʿ; and it is sufficient that in every qunūt he says ‘subḥānal lāhi wal ḥamdu lillāh’. If there is not enough time, he must only say the takbīrs; and if there is not enough time to perform the takbīrs, it is sufficient that he follows the imam and goes into rukūʿ.