He was listening

Abu Musa was reciting at night. He was not performing and there was no gathering; he was reading, in the dark, on his own.

The Prophet ﷺ was passing, and stopped, and listened.

He said nothing at the time. Abu Musa found out the next day, when he was told that he had been listened to — and that if he had known, he would have made it more beautiful still.

What he was told was that he had been given a share of the music of the house of Dawud.

It is a small scene and it carries a lot. The reciting was private, which is why the praise means what it means — nobody adjusts for an audience they do not know is there. And the compliment reaches backwards, past the Arabs entirely, to a prophet whose psalms are the standing reference for what beautiful praise sounds like.

Abu Musa spent the rest of his life teaching people to read. Two of the largest cities in the Muslim world learned the Qur'an substantially through him. This is where that starts, and it starts with him not knowing anyone could hear.

The report is in both Sahih collections.