Faithful witness in Greek:
Ho martys, ho pistos
Strong’s Concordance numbers: 4103, 3144
Bible reference: Rev. 1:5
John began his book of Revelation with these words:
John to the seven churches that are in Asia: Grace to you and peace, from Him who is and who was and who is to come, and from the seven Spirits who are before His throne, and from Jesus Christ, the faithful witness, the firstborn of the dead, and the ruler of the kings of the earth. To Him who loves us and released us from our sins by His blood. (Revelation 1:4-5)
Jesus (Yeshua‘) used a similar name for Himself, Faithful and true witness (Rev. 3:14).