Asslaamu alaykum. Abu Harb ibn Abu Al-Aswad reported on the authority of his father that Abu Musa Al-Ash'ari sent for the reciters of Basra. They came to him, and they were three hundred in number. They recited the Qur'an, and he said:
“You are the best among the inhabitants of Basra, for you are the reciters among them. So continue to recite it. (But bear in mind) that your reciting for a long time may not harden your hearts as the hearts of those before you were hardened. We used to recite a surah which resembled in length and severity to (Surah) Bara'at. I have, however, forgotten it with the exception of this which I remember out of it: "If there were two valleys full of riches, for the son of Adam, he would long for a third valley, and nothing would fill the stomach of the son of Adam but dust." And we used so recite a surah which resembled one of the surahs of Musabbihat, and I have forgotten it, but remember (this much) out of it: 'Oh people who believe, why do you say that which you do not practise.' (lxi 2.) and, 'That is recorded in your necks as a witness (against you) and you would be asked about it on the Day of Resurrection" (xvii. 13). Shaykh, please explain the above hadith about Abu Musa forgetting that surah. What surah was it?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Some scholars held that the two soorahs to which this hadeeth referred were abrogated, as stated by Al-Qurtubi in his book Al-Mufhim.

Some scholars, on the other hand, such as Al-Maaziri, maintained that it is also possible that these soorahs were not abrogated but that Abu Moosa forgot them. It is mentioned in the book titled Al-Mu'lim that as for the narrator's statement, “'We used so recite a soorah which resembled one of the soorahs of Musabbihaat (i.e. soorahs nos. 57, 59, 61, 62, 64 and 87), and I have forgotten it;' it is possible that he was referring to one of the soorahs that are recited now but forgot it and only remembered the abrogated verse in that soorah.”

This view is supported by some reported ahaadeeth indicating that the statement reading, “If there were two valleys full of riches for the son of Aadam (Adam), he would long for a third valley, and nothing would fill the stomach of the son of Aadam but dust,” was actually part of soorah Al-Bayyinah (98). It has been narrated on the authority of Kaʻab ibn Maalik that the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, said to him, “Indeed, Allaah has ordered me to recite the Quran to you.” So he, sallallaahu ʻalayhi wa sallam, recited the verse that reads (what means): {Those who disbelieved among the People of the Scripture and the polytheists were not to be parted (from disbelief) until there came to them clear evidence.} [Quran: 98:1] and he recited in it, "Indeed the religion with Allaah is the Muslim Hanifiyyah, not Judaism nor Christianity, whoever does good then it shall not be rejected from him.” Then he, sallallaahu ʻalayhi wa sallam, recited, “If the son of Aadam had a valley of money, he would seek a second, and if he had a second, he would seek a third, and nothing fills the belly of the son of Aadam except dirt. And Allaah pardons those who repent.” [Sunan At-Tirmithi]

Abu ʻEesa At-Tirmithi classified this hadeeth as hasan saheeh (good or sound).

In any case, we should not mistakenly assume that any part of the Quran went missing based on this hadeeth and its like; verily, this is plain falsehood, as evidenced by the words of Allaah: {Indeed, it is We Who sent down the Quran and indeed, We will be its guardian.} [Quran 15:9] Moreover, it has been established by the consensus of the Companions and the following Muslim generations that the Quran that we are enjoined to worship Allaah by reciting it and acting upon its teachings is that which is between the two covers of the mus-haf, without any addition or omission, as asserted by Al-Qurtubi in his book Al-Mufhim.

Allaah knows best.