A First Example: 1 Samuel 2.27->
This is not an entirely coherent book, more of an example of how different sacred legends were later pasted together, leaving more than a few inconsistencies.
Anyway, we've just been given the story of Samuel's mother Hannah, long barren in the typical Bible birth tradition, who goes to the sanctuary at Shiloh to pray for a son. Eli, the priest in charge, adds his blessing to the prayer and she soon conceives, so that she carries out her promise and dedicates Samuel to service at the sanctuary when he is quite young. Eli's own sons, meanwhile, are misbehaving, scrounging the tasty bits of people's sacrifices & harassing the women participating in the services.
And there came a man of God to Eli, and said to him, "Thus the Lord has said, 'I revealed myself to the house of your father when they were in Egypt subject to the house of Pharoah.
'And I chose him out of all the tribes of Israel to be my priest, to go up to my altar, to burn incense, to wear an ephod before me; and I gave to the house of your father all my offerings by fire from the people of Israel. Why then look with greedy eyes at my sacrifices and my offerings which I commanded, and honor your sons above me by fattening yourselves upon the choicest parts of every offering of my people Israel.'
Therefore the Lord the God of Israel declares: 'I promised that your house and the house of your father should go in and out before me forever,' but now the Lord declares: 'Far be it from me, for those who honor me I shall honor, and those who despise me shall be lightly esteemed.
'Behold, the days are coming, when I will cut off your strength and the strength of your father's house, so that there will not be an old man in your house. Then in distress you will look with envious eye on all the prosperity which shall be bestowed upon Israel; and there shall not be an old man in your house forever. The man of you whom I shall not cut off from my altar shall be spared to weep out his eyes and grieve his heart; and all the increase of your house shall die by the sword of men. And this which shall befall your two sons, Hophni and Phinehas, shall be the sign to you: Both of them shall die on the same day.
'And I will raise up for myself a faithful priest, who shall do according to what is in my heart and in my mind; and I will build him a sure house; and he shall go in and out before My anointed forever.
'And everyone who is left in your house shall come to implore him for a piece of silver or a loaf of bread, and shall say, "Put me, I pray you, in one of the priest's places, that I may eat a morsel of bread."'"
3.1) Now the boy Samuel was ministering to the Lord under Eli. And the word of the Lord was rare in those days; there was no frequent vision.
Who was that anonymous man? If the references to an 'anointed' and a new 'house' for God are anachronistic, the gist of what he's saying is quite plausible, and typical of what later prophets had to say, ie warnings that someone in power was misusing it, an implied cease-and-desist, a prediction of trouble to come if the abuse continued.
And while the man may well belong to one of the bands of prophets we find mentioned later, we're also told that such messages are not common at the time.