From the Folklore of the Ao Naga
Once upon a time, all men complained of the Sun’s heat. This made the Sun very angry, for he did not like hearing his name bandied about from mouth to mouth. So one evening he set as usual and refused to return from the underworld in the morning. The earth was dark and all its inhabitants were in despair. Vain embassies from men, animals, and birds asked the sun to return. But he would listen to none of them. At last they all beseeched the cock to go and see if he could persuade the sun to come and shed his light once more. The cock agreed to go, but reluctantly, for he was very afraid of being eaten by a jungle cat on his way to the underworld.
He reached the sun’s presence and tried to persuade him to return, saying, “You have six doors to come through as you rise every morning. As you open each I will crow, that all men may know you are coming.” But the sun hardened his heart and refused to come. At last the cock spake as follows, “I have come so far to see you, you must at least promise me this: If I am attacked by a jungle cat on my way back, I will crow and you must come to my rescue.” To this the sun agreed and the cock went on his way. When he had gone a short distance he crowed, though there was no jungle-cat anywhere near. True to his promise, the sun came from the underworld to help him. That is why the sun rises every morning when the cock crows.
Sources:
Chapter 5, The Ao Nagas, Mills
Kotharian Lightning 