With this statement Peter is referring to himself, James, and John. A group of people were eyewitnesses and thus it is not something made up by one person.1 They were on the mountain to see what happened to Christ and hear the voice from heaven. Seeing and hearing are what true witnesses of an event have experienced, which is why Peter has drawn attention to both.2
18 we ourselves heard this very voice borne from heaven, for we were with him on the holy mountain.