Determination of a Surah (for recitation in a Salaah)
In Hujjatullah-al Baligha, Hadrat Shah Waliullah (May ALLAH bless him) has stated as follows: The Messenger of ALLAH chose to recite a particular sürah in a particular Salaah on certain conditions and expediencies but neither fixed them positively nor enjoined on others to do so. As such if any one follows his sunnah (and recite the same surah in the Salaah) it is in order, and if one does not do so, it does not matter. [Ma’rif -ul- Hadis]