1 Samuel 21:7

Now a certain man of the servants of Saul was there that day, detained before the Lord; and his name was Doeg, an Edomite, the chiefest of the herdmen that belonged to Saul.

Cross-reference

1 Samuel 22:9 shows Doeg reporting David's visit to Saul — the same Doeg introduced here, continuing the narrative.

Psalm 52 is a maskil of David about Doeg's betrayal — directly referencing the event where Doeg informed on David.