4 Nephi 1:48

And it came to pass that when three hundred and twenty years had passed away, Ammaron, being constrained by the Holy Ghost, did hide up the records which were sacred; yea, even all the sacred records which had been handed down from generation to generation, which were sacred, even until the three hundred and twentieth year from the coming of Christ.

When 320 years had passed, the Holy Spirit told Ammaron to hide the sacred records. He hid all the sacred records that had been passed down from group to group. He did this in the 320th year since Christ had come.

When 320 years had passed, the Holy Spirit compelled Ammaron to hide the sacred records—all the sacred writings that had been passed down through the generations since Christ’s coming.