Tafsīr al-ʿAyyāshi: From Yūnis bin ʿAbd al-Raḥmān, from the one who narrated in a marfūʿ fashion, who said:

I asked Abā ʿAbd Allāh عليه السلام [about the āya]: Indeed we have given you the seven often repeated verses and the Glorious Qur’ān.1
He said: It is the chapter of al-Ḥamd (i.e. al-Fātiḥa), and it’s seven verses, from it is: In the name of Allāh, The Entirely Merciful, The Especially Merciful (Bismillah al-Raḥmān al-Raḥīm). It is only named al-Mathāni because it’s recited paired (i.e. recited twice) in the two cycles (rakʿa) of prayer.2

1. al-Ḥijr (15:87)
2. al-Fātiḥa is recited in the first cycle, and second cycle, of all the daily obligatory prayers.

عن يونس بن عبدالرحمن ، عمن رفعه ، قال : سألت أبا عبدالله ( عليه السلام ) : ولَقَدْ آتَيْناكَ سَبْعاً مِنَ الْمَثانِي والْقُرْآنَ الْعَظِيمَ ؟ فقال : هي سورة الحمد ، وهي سبع آيات منها * ( بِسْمِ اللَّه الرَّحْمنِ الرَّحِيمِ ) * وإنما سميت المثاني لأنها تثنى في الركعتين