For a detailed discussion, see Metzger, Textual Commentary, 2nd ed., 647-49.
As Dr. Daniel Wallace observes:
“There is no sure evidence of this reading in any Greek manuscript until the 1500s; each such reading was apparently composed after Erasmus’ Greek NT was published in 1516. Indeed, the reading appears in no Greek witness of any kind (either manuscript, patristic, or Greek translation of some other version) until AD 1215 (in a Greek translation of the Acts of the Lateran Council, a work originally written in Latin).https://bible.org/article/textual-problem-1-john-57-8 (accessed 3/26/2018).