The Acts of the Apostles, while not included in the gospel accounts, introduces itself as a continuation of Luke's gospel written by Luke and addressed to the same named recipient:
Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us, Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the word; It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus, That thou mightest know the certainty of those things, wherein thou hast been instructed. - Luke 1:1-4
The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach, Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen: - Acts 1:1-2
At Acts chapter 16:10 when Luke joins Paul in his journeys many of the pronouns which had been in the third person (they) switch over to the first person (we) and continue that way for the rest of the Acts:
And after he had seen the vision, immediately we endeavoured to go into Macedonia, assuredly gathering that the Lord had called us for to preach the gospel unto them. - Acts 16:10