Polycarp's Letter to the Philippians is referred to by Irenaus as follows: 'There is also a forceful epistle written by Polycarp to the Philippians, from which those who wish to do so, and are anxious about their salvation, can learn the character of his faith, and the preaching of the truth.' as quoted in 'A new Eusebius' edited by J Stevenson. [pub SPCK London, 1965] page 120

text to be found at http://www.earlychristianwritings.com/text/polycarp-lightfoot.html