There is a tradition in the Eastern Orthodox Church that the saying was made popular by Mary Magdalene when she supposedly addressed Emperor Tiberius in Rome with the words “Christ is risen.” Mary Magdalene witnessed most of the events surrounding the crucifixion. She was present at the mock trial of Jesus; she heard Pontius Pilate pronounce the death sentence; and she saw Jesus beaten and humiliated by the crowd. She was one of the women who stood near Jesus during the crucifixion to try to comfort Him. The earliest witness to the resurrection of Jesus, she was sent by Jesus to tell the others (John 20:11-18).
The greeting is based on Luke 24:34. Translations throughout church history, from the Latin Vulgate (c. AD 400) to the ESV (2001) have translated this verse nearly identically: “The Lord has risen indeed, and has appeared to Simon!” See also Matthew 28:1-9 which describes the resurrection events as witnessed by Mary Magdalene and the other women who were present.
The Russian Orthodox Church says this about the Paschal Greeting:
“Do you know the origin of this custom? It is quite an ancient one. According to tradition, it began with the Equal to the Apostles St. Mary Magdalene. After the Lord’s Ascension, she came to Rome to preach the Gospel. Standing before Emperor Tiberias and saying, “Christ is Risen!” she presented him with a red egg. In those days, it was customary for poor people to give their friends, benefactors, the wealthy, and authorities, an egg on the New Year and on birthdays, as a sign of respect. Following Mary Magdalene’s example, early Christians began giving eggs to one another on the days celebrating Christ’s Glorious Resurrection. They passed that custom on to us.” 
Apparently the words “He is risen” were spoken at Augustine's baptism on Easter Eve AD 387. The Paschal Homily of St John Chrysostom (ordained AD 386 and made Patriarch or Archbishop of Constantinople in 386) may be the origin of the Paschal Greeting. It was written circa 400 AD but says nothing about Mary Magdalene or Emperor Tiberius, or the giving of eggs.  See also ‘The Origin and Meaning of the Paschal Greeting (April 2007’) – link below.
It may be worth checking on The Gospel of Mary, which was discovered in the Akhmim Codex in Cairo, Egypt, in 1896. It was not made public until 1955, when it was published due to the popularity of the Nag Hammadi library. Written in Greek and Coptic, the Gospel of Mary codex is dated to the 3rd (Greek) and 5th (Coptic) centuries A.D. The gospel of Mary is mentioned in the writings of some of the early church fathers as early as the 3rd century A.D. In the only known copy of the text, ten entire pages are missing, including the first six pages. As a result, it is difficult to arrive at a coherent and consistent overall message. The gospel of Mary was not written by Mary Magdalene or any other Mary of the Bible. The Gnostic teachings found in the gospel of Mary date it to the late 2nd century A.D. at the earliest. It may say something about Mary Magdalene going to Rome to preach to Emperor Tiberius. However, I have no intention of digging into that spiritual can of worms!
Other possible source of information: “Christ is risen!” Resp. “He is risen indeed! Alleluia!” A Dictionary of Biblical Tradition in English Literature (Grand Rapids, MI: W.B. Eerdmans, 1992).