They obeyed to receive the promise
The 120 disciples, including the apostles, obeyed Jesus’ command to wait until they received. Jesus promised they would receive the Promise of the Father, the Helper, the Spirit of Truth, the Baptism of the Holy Spirit.
They had heard all these descriptive statements, and yet the apostles did not know if they were different experiences to be received at different times or various names describing the same experience that would be received during one event. It proved to be one experience received at one time when they all began to speak in tongues.
From then on the apostles prayed for people to receive the gift of their own spirit languages and established speaking in tongues as proof that a person had received the gift of the Holy Spirit (see John 14:16–17, 26; 16:7; Luke 24:49; Acts 1:4).