These familiar words were spoken by the Lord Jesus in His so-called "sermon" on the mount—actually a set of instructions primarily addressed to disciples (note Matthew 5:1-2). Jesus often spoke about your father in heaven, when speaking to a number of His disciples together (e.g., Matthew 18:10, and many others, with our text above being the first). He also called Him "thy father" when speaking to an individual believer (e.g., Matthew 6:4,6—the King James translation follows the Greek original in thus distinguishing between second person singular and plural pronouns).
When talking about His own unique relation to the heavenly Father, on the other hand, the Lord Jesus spoke directly of my Father (e.g., Matthew 7:21; 10:32). Although all born-again believers are "sons of God" (John 1:12), the Lord Jesus Christ is the "only begotten Son" of God (John 3:16), "whose goings forth have been from of old, from everlasting" (Micah 5:2).
The distinction between His relation to the heavenly Father and ours is especially noted in the first words Jesus uttered after His resurrection. Mary Magdalene was the first believer to see Him after He returned from the grave. When she recognized Him and perhaps started to touch Him, He said: "Touch me not; for I am not yet ascended to my Father" and then He said: "I ascend unto my Father, and your Father" (John 20:17). He did not say "ascend to our Father." Our own relation to the Father in heaven is beautifully typified by the relation we should have to our earthly fathers. And, just as a good son desires to honor his human father in word and deed, so Jesus exhorted us to glorify our heavenly Father by our lives here on earth. HMM