Acts 1:1

Amplified Bible

1 The first ^[a]account I made, Theophilus, was [a continuous report] about all the things that Jesus began to do and to ^[b]teach^(A)

Acts 1:1 Luke is referring to his gospel.
Acts 1:1 As the gospel tells us what Jesus began to do and teach, so Acts tells what He continued to do and teach by His Holy Spirit (F. F. Bruce).

Acts 1:1

New International Version

1 In my former book,^(A) Theophilus, I wrote about all that Jesus began to do and to teach^(B)

Amplified Bible (AMP)

New International Version (NIV)