This question already has an answer here:
Thank you very much. If I meet imam in the 3rd Rakat (Ishai prayer-where 3rd and 4th rakats are only Faatihat rakats) and of course complete the 4th rakat with him. Now i have done only 2 rakats. To pray my 3rd and 4th rakats should i read Faatihat only (in the 3rd and 4th rakat respectively) or i should read surat after the Faatihat in the 3rd and also in the 4th rakat. Not reading surat beside faatihat in this case makes all my ishai rakats only "faatihat rakats". Is that OK?