Here are some quotes and references I came across, hope it helps you. There is much more in this link: The Intercession of Saints
"Remember me, you heirs of God, you brethren of Christ; supplicate the Savior earnestly for me, that I may be freed through Christ from him that fights against me day by day" (The Fear at the End of Life [A.D. 370]).
"By the command of your only-begotten Son we communicate with the memory of your saints . . . by whose prayers and supplications have mercy upon us all, and deliver us for the sake of your holy name" (Liturgy of St. Basil [A.D. 373]).
Also, try Praying to the Saints
Both are Catholic.com resources. I personally have found that they have a good library of apologetics. The reasoning is that some of their apologists were devout Protestant Christians, and slowly began challenging their beliefs until they came to Catholicism. They use the angle that while early writings of the church may not be infallible, it is still helpful for insightful understanding of what the church believed at the beginning.