This story is narrated neither in Maanas, nor in Raamaayan, but it is told. It is mentioned in Hanumaan Aaratee: "Paithi Pataal Tori Yam Kaarey, Ahiraavan Kee Bhujaa Ukhaarey". And it is mentioned in Hanumaanaashtak also: 

"Bandhu Samet Jabai Ahiraavan Laay Raghunaath Pataal Sidhaaro;
Devahin Pooji Bhalee Vidhi San Bali Deu Sabahi Mili Mantra Uchaaro;
Jaay Sahaay Tabahee Ahiraavan Sen Samet sanhaaro; 
Ko Nahin Jaanat Hai Jag Mein Kapi Sankat Mochan Naam Tihaaro."

 I write this story just for information. Any addition, subtraction or comments about it are welcomed?

Once, in the night, when all were sleeping, Ahiraavan lifted sleeping Raam and Lakshman from their camp and brought them to Lankaa Puree. He started doing Yagya in which he wanted to offer their sacrifice.

When Vaanar came to know about the absence of Raam and Lakshman, they got worried. Then Vibheeshan expressed his doubts on Ahiraavan. Now who should go there and bring them back safely. Again Hanumaan went there in the form of an insect hiding in flowers which were taken for Ahiraavan's worship by one of his maids. After reaching there Hanumaan changed himself to a huge size, killed Ahiraavan and brought unconscious Raam and Lakshman back to camp.

This is another version of Maanas version of that incident in which Naarad Jee sent Garud to help Raam, (see Raam). In this version Naarad Jee doesn't send Garud Jee, but Hanumaan goes to bring him for this purpose and it is given in Raamaayan.

Once Meghnaad used Naag Paash on Raam and Lakshman. They both got tied and got unconscious. Naag (serpents) started biting them slowly and their poison started spreading in their body bit by bit. If nothing was done soon the poison would have spread in the whole body, and nobody could save them.

All Vaanar were just standing around them sad and watching them dying slowly. Nobody could do anything. After some time Hanumaan was looked for, but he was nowhere to be found and seen. Now Vaanar were more worried as they trusted Hanumaan very much.

In fact Hanumaan had already gone to Garud Jee to request him to help Raam in His difficult times. Garud Jee readily agreed and came there where Raam and Lakshman were lying. As soon as serpents saw Garud Jee, they ran away and thus Raam and Lakshman's lives were saved. Everybody admired Hanumaan's presence of mind.