A Summary of the Srimad Bhagavatam : Ch-1. Part-1.
1: King Parikshit’s Question to Suka Maharishi : 1.
Asvatthama obtained his astra, which is known as Narayana Astra, as a special gift from his father Drona when he insisted that he should be given something which Arjuna did not know – because, naturally, it should be accepted that a disciple is not as great as one’s own son. Due to this persistence, Drona bestowed an indomitable power known as Narayana Astra upon Asvatthama, knowing well that the boy was mischievous and was likely to use it unwarrantedly. Drona warned him that it should not be used recklessly, yet he knew that he would not listen to his advice. So, as a safeguard, he did not teach him the art of using it a second time or the art of withdrawing it. It could be discharged once only, and then it would extinguish itself.
When, to Asvatthama’s consternation, the attempt to destroy the Pandavas with this missile failed, he ran away from the field cursing everybody and yelling out that even fathers are not to be trusted these days because his father duped him, as it were, by saying that he had initiated him into an invincible astra, which actually amounted to nothing. When Asvatthama was shouting like this while running away, he met Vyasa – the great Krishna Dvaipayana – on the way, who asked him what the matter was. He said, “My father did not tell me the truth. I was initiated into the invincible force called Narayana Astra, being told that no one on Earth can stand before it; but when I used it, nothing actually happened.” The great Vyasa replied, “My dear boy, your father has not made any mistake. He gave you that strength which no one else in the world could wield. But you used this astra of Narayana against Narayana Himself. Therefore, it would not work.”