The answer for this question and countless questions like it regarding the source of facts in the Gospels and other New Testament writings is actually similar to the answer to the question: “How does any biblical author know the information they communicate to us?” Recently I read Brad Meltzer’s book, The First Conspiracy. This book is about a conspiracy to assassinate General George Washington. The facts are found in various ancient documents, but it took a researcher to dig them out and connect them into readable story. The researcher called the story to Meltzer’s attention who ably wrote out the story form. Similarly, Matthew would have researched the facts he shared. He would have been assisted by the other disciples of Jesus who carefully assembled the facts surrounding the crucifixion and resurrection of Jesus. Also, Matthew had interest in aspects of Jesus’ story that fulfilled Hebrew prophecy. Moreover, Matthew had the leadership of the Holy Spirit as he wrote his Gospel. The fact that Matthew did not footnote his sources should not be a concern to us because we have the implied affirmation of the disciples/apostles of Jesus and the early church who could have spoken out if Matthew’s work had not been truthful. In fact, ancient writers did not follow the modern academic standard of declaring sources. In fact, read a non-fiction book by a credible scholar from 150 years ago and see how they handled sources. You may have the same question for them! What is far more important is the high standing Matthew’s Gospel has enjoyed throughout the years, first by the apostles and then by early Christians who had access to ancient Palestine and the persons named in the Gospels. The early Christians accepted the historical accuracy of Matthew and affirmed spiritual quality of the truth it conveys. Therefore, we have good reason to be confident in the truthfulness of Matthew’s Gospel, even if we don’t have outside affirmation for the 30 pieces of silver or other facts whose sources are not noted.