Wa alaykum salam wa rahmatuLlahi wa barakatuHu,
For one who is able, reciting Surah al-Fatihah is obligatory. And this is the case when one is praying alone, behind an imam, or as the imam. As well, it is the case when the prayer is audible or inaudible, obligatory or supererogatory. And this applies to every rak’ah of prayer except for when one is a latecomer. (Rawdah al-Talibin 1/241)
And Allah knows best.
Answered by: Shaykh Yaqub Abdurrahman