Reciting a surah after surah faatiha before going into ruku
Q: I was praying maghrib. After reciting Surah Faatiha in the first rakah I forgot to add another surah and was going for ruku. I opened my arms but just when I said "Alla-"(of Allah Akbar) I realised my mistake. I once again joined my arms and recited the second surah and did rest of the salaah normally. I did not do sajda sahw. Is my prayer valid?
Also is sajda sahw valid for all mistakes in salaah or is it invalid if you missed fards of salah?