I flagged this answer as not an answer, which has now been declined.
The question is asking for evidence of the first (or early) examples of Christians who prayed/asked the saints to intercede for them.
This answer gives a quote, and here's the full version from the website it links to:
If the Apostles and Martyrs, while still in the body, can pray for others, at a time when they must still be anxious for themselves, how much more after their crowns, victories, and triumphs are won! One man, Moses, obtains from God pardon for six hundred thousand men in arms; and Stephen, the imitator of the Lord, and the first martyr in Christ, begs forgiveness for his persecutors; and shall their power be less after having begun to be with Christ? The Apostle Paul declares that two hundred three score and sixteen souls, sailing with him, were freely given him; and, after he is dissolved and has begun to be with Christ, shall he close his lips, and not be able to utter a word in behalf of those who throughout the whole world believed at his preaching of the Gospel? And shall the living dog Vigilantius be better than that dead lion? ("Contra Vigilant.", n. 6, in P.L., XXIII, 344).
This quote is saying that the dead saints can pray/intercede for us, but it is not evidence that Jerome prayed/asked them to intercede or encouraged others to do so. It just doesn't mention it!
As comments on the answer show, at least 5 people think this is not an valid answer.