When is the first documented case of Christians praying to the dead saints? When is the first documented case of Christians praying to the dead saints?

(A point in comments was brought up that "asking the dead saints" does not mean "praying to the dead saints" as "prayer is always addressed to God only". For this case, the phrase "praying to the saints" in my question can be replaced then with "communicating to saints" or "addressing the saints" in hopes that those whom they address will somehow hear them.)
The earliest may not be easy to find, but I'll put a marker down.
St Ephraim the Syrian, who died in 373, prayed:

Ye victorious martyrs who endured torments gladly for the sake of the God and Saviour; ye who have boldness of speech towards the Lord Himself; ye saints, intercede for us who are timid and sinful men, full of sloth, that the grace of Christ may come upon us, and enlighten the hearts of all of us that so we may love him.
— Encomium in Martyres

Here he is clearly invoking the prayers of martyrs (who have died, rather by definition).