While Hanuman was flying across the ocean toward Lanka, the Devas, Gandharvas, Siddhas, and great sages decided to test his strength, courage, and intelligence. They approached Surasa, the mother of Nagas, and said: “This glorious son of Vayu, Hanuman, is flying over the ocean. You obtain a terrifying mountain like demonic form with fearful tusks, red-brown eyes, and a mouth as vast as the sky. Create an obstacle for him for a moment. We wish to see whether he can overcome you through strength, courage, and intelligence.”

Then Surasa in a dreadful form in the middle of the ocean and blocked Hanuman’s path. She declared:

Verse : mama bhakṣyaḥ pradiṣṭastvam īśvarair vānararṣabha |ahaṃ tvā bhakṣayiṣyāmi praviśēdaṃ mamānanam || 150 ||

“O best among Vanaras! The gods have given you to me as food. Enter my mouth.”

Hanuman respectfully replied: “Rama, the son of Dasaratha, entered the Dandaka forest along with his brother Lakshmana and wife Sita. While Rama was occupied elsewhere, Ravana abducted Sita. I am going as Rama’s messenger to search for her. Please allow me to complete Rama’s mission.

Verse : athavā maithilīṃ dṛṣṭvā rāmaṃ cāklisṭakāriṇam | āgamiṣyāmi tē vaktraṃ satyaṃ pratiśṛṇōmi tē || 155 || 

After seeing Sita and informing Rama, I promise truthfully that I shall enter your mouth.”

Surasa answered, “No one can pass me without entering my mouth. This is a boon granted to me by Brahma.”

Saying this, she opened her enormous mouth before Hanuman. Hanuman then expanded his body to ten yojanas in length and width. Seeing this, Surasa widened her mouth to twenty yojanas. Hanuman increased himself to thirty yojanas. Surasa enlarged her mouth to forty yojanas. Hanuman became fifty yojanas in size. Surasa stretched her mouth to sixty yojanas. Hanuman grew to seventy yojanas. Surasa widened her mouth to eighty yojanas. Hanuman then became ninety yojanas tall like a mountain. Surasa expanded her mouth to one hundred yojanas.

Seeing her vast mouth opened wide with a terrible tongue and fearful appearance, Hanuman suddenly reduced himself to the size of a thumb. With great speed, he entered Surasa’s mouth and immediately came out again, standing in the sky.

Hanuman then said, “O daughter of Daksha! I have indeed entered your mouth. Your boon has been fulfilled. Now permit me to continue my journey to Sita.”

Surasa resumed her original form and said, “O gentle Hanuman! Go and successfully accomplish Rama’s task. May you reunite Sita with Rama.”

That Hanuma, equal to Garuda in speed, neared the ocean which is the abode of Varuna, entered the sky and went in His original path.

The Devas and sages praised Hanuman for overcoming the challenge through intelligence, courage, and presence of mind. This incident is mentioned in Valmiki Ramyana - Sundarakanda - sarga 1 - verses 144 - 170