SECTION ON PRAYER · 134 – Reciting a Surah in Two Rak'ahs
Muadh ibn Abdullah al-Juayni (may Allah be pleased with him) relates: A man from the tribe of Juayna reported to me that he heard the Messenger of Allah (peace and blessings upon him) recite [Surat] Az-Zalzalah in each two rak'ahs of the Fajr prayer. I do not know whether the Messenger of Allah (peace and blessings upon him) forgot or knew that he was reciting it.
It is disliked for one who knows another verse to repeat the same verse in the same rak'ahs [of a prescribed prayer]. However, if he does not know any other verse, then it is not disliked. It is not disliked in voluntary (nafl) prayers.