The story of Mata Sita gifting a necklace to Hanuman is found in the Yuddha Kanda of Ramayana. This incident reveals the greatness of Hanuman.
Bhagavan Sri Ramachandra gave Mata Sita a pearl necklace,
glittering like the moonbeams, along with bright garments and beautiful
ornaments. Sita looked at them, and then gave them to Hanuman. Next, removing
her own necklace she looked repeatedly at the assembled vanaras as well as at
her husband. Rama, understanding her intent, told her to give that to the one
with whom she was most pleased. Sita gave the necklace to Pavanaputra Hanuman,
who was possessed of [such ‘pearl-like’ attributes as] energy, fortitude,
glory, dexterity, efficacy, humility, statesmanship, valor, prowess, and
discernment. Hanuman wore the necklace and shone like a mountain lighted up by
the moonbeams. (Valmiki Ramayana, 6.131.78–83.)