Matali, the charioteer of Indra, becomes the charioteer of Lord Rama.

Valmiki Ramayana, Yuddha Kanda, Chapter 102, Verses 5 to 19:

Ravana for his part, standing in his chariot struck Rama with thunderbolt-like arrows, as a cloud strikes gushes of rain on a huge mountain. With an attentive mind, Rama showered golden decked arrows, looking like blazing flames of fire, on Ravana in the battle-field. The gods, celestial musicians and kinnaras the mythical beings thought that the battle was not equal between Rama standing on the ground and Ravana fighting from a chariot.

Hearing their nectar-like words, the illustrious Indra, the chief f celestials then called Matali and spoke as follows: "Go quickly with my chariot to Rama, who is standing on the earth. After reaching the earth, invite him to take his seat in the chariot and carry out a great service to the gods."

Hearing the words of Indra, Matali, the charioteer of Indra, offering salutation to him by bowing his head, spoke then the following words:
"O Indra! Providing with green horses the excellent chariot, I will proceed then immediately and perform the duty of a charioteer to Rama.”

Then came the glorious and excellent chariot of Indra, having variegated body decked in gold, which was fitted with hundreds of tiny bells, with its pole of cat's eye gems shone like the morning sun, yoked to excellent green horses, having golden chaplets on their heads, having white whisks, covered with nets of gold, shining like the sun and bearing a flag-staff raised on a golden bamboo. Ascending the chariot, as enjoined by Indra and descending from paradise, Matali approached Rama.

Then Matali, the charioteer of Indra, along with a whip in his hand, stayed in the chariot itself and joining his palms in salutation, spoke the following words to Rama. "Indra has sent this chariot to you for your victory, O the mighty and the glorious Rama, the destroyer of adversaries! Here is the great bow belonging to Indra, an armour shining as fire, some arrows as bright as sun and a stainless auspicious spear. O heroic king! Mounting this chariot with me as a charioteer, kill Ravana the king, as earlier Indra the lord of celestials killed the demons."

Duly circumambulating that chariot (as a mark of respect) and offering his salutation, when thus spoken by Matali, Rama then ascended the chariot, causing the three worlds to shine forth with his splendour. Then ensued a tumultuous and thrilling battle, between the great armed Rama and Ravana, the demon.