Surya, or Sun God, was the Guru of Hanuman. But Surya put a strange condition if Hanuman was to take lessons from him. As per this story Hanuman had to walk backwards facing the sun.
The sun god had offered to become Hanuman’s tutor. When the
latter approached him, Surya put
a condition. Since Surya had to keep moving, Hanuman would have to keep walking
with his face towards the Sun. Hanuman accepted the condition. With his book open in his hands,
his eyes fixed on
the Sun, Hanuman kept walking backwards in the sky, synchronizing his steps
with the Sun’s movement. In this way, he mastered grammar and other academic
disciplines. (Valmiki Ramayana, 7.36.45; Tulsidas, Hanuman Bahuk, 4.)
On completing his education with Surya, Hanuman insisted on offering his guru-dakshina, the preceptor’s fee. Surya asked him to serve Sugriva, his son, who was not as strong and powerful as Vali, the chief of the vanaras. This brought Hanuman into Sugriva’s service.