A man from Juhaynah informed him that he heard the Prophet ﷺ recite "When the earth is shaken" [Zalzalah 99:1] in the morning prayer, in both of its two units. He said, "I do not know whether the Messenger of Allah ﷺ forgot or recited it intentionally."
أن رجلا من جهينة أخبره أنه سمع النبي ﷺ يقرأ في الصبح: {إذا زلزلت الأرض} في الركعتين كلتيهما، فلا أدري أنسي رسول الله ﷺ أم قرأ ذلك عمدا.
Abu Dawud (816) narrated it from Ahmad bin Salih, who said, Ibn Wahb told us, Amr told me, from Ibn Abu Hilal, from Muaz bin Abdullah Juhani, and he related the same account. Its chain is hasan due to some discussion regarding Ibn Abu Hilal, who is Saeed bin Abu Hilal Laythi, their freedman, Abu Ala Misri, though some say he was from Madinah, and regarding his teacher Muaz bin Abdullah Juhani, though both of them are "truthful." Nawawi said in Khulasa (1226), "Abu Dawud narrated it with a sahih chain."As for the companion's statement, "I do not know whether the Messenger of Allah ﷺ forgot or recited that intentionally," the basic principle is that an action of the Prophet ﷺ is considered to be a legislated practice, and when a companion is uncertain between forgetfulness and intention, the ruling favors intention unless evidence proves otherwise. I have not come across anything prohibiting the repetition of a single surah in the two rakahs. This is why Abu Dawud and others titled a chapter, "Chapter on a man repeating a single surah in the two rakahs."