We have the words of witnesses who, on two occasions, heard God speak with a voice that could be understood by people, confirming that Jesus was his son.
The first of these occasions was Jesus’ baptism:
…suddenly the heavens were opened to him and he saw the Spirit of God descending like a dove and alighting on him. And a voice from heaven said, “This is my Son, the Beloved, with whom I am well pleased.” (Matthew 3:13-17 – See also Mark 1:9-11; Luke 3:21-22.)
The second occasion was on the mountain where Jesus was transfigured. On this occasion, God also commanded that the people who heard him should listen to Jesus:
…suddenly a bright cloud overshadowed them, and from the cloud a voice said, “This is my Son, the Beloved; with him I am well pleased. Listen to him!” (Matthew 17:1-8 – See also Mark 9:2-8; Luke 9:28-36.)
The apostle Peter was with Jesus on the mountain on this second occasion and confirmed that he and his companions heard God acknowledge Jesus as his son:
“For he received honor and glory from God the Father when that voice was conveyed to him by the Majestic Glory, saying, ‘This is my Son, my Beloved, with whom I am well pleased.’ We ourselves heard this voice come from heaven, while we were with him on the holy mountain.” (2 Peter 1:17-18)