The yawn of Vishnu and fight with Shiva is found in the Bala Kanda chapter 1 (74) in the Ramayana. The story was narrated by Parashurama to Bhagavan Sri Rama. Parashurama was describing about the two bows constructed by Vishwakarma. One which was originally in the possession of Shiva was broken by Sri Rama to marry Mata Sita. The other was originally in the possession of Vishnu and it was now held by Parashurama.
Once the Gods and Rishis wanted to find whose bow was powerful. To ascertain the strength of their bows, Shiva and Vishnu entered into a duel. There was a great encounter between Shiva and Vishnu, as they sought to defeat each other.
Finally through his yawn and humkara, Vishnu was able to counter the terrible and excellent bow of Shiva.
Thus it was decided that the bow of Vishnu was superior.
Shiva then gave away his bow to Rajarshi Devarata of Videha. Vishnu gave his bow to Richika Bhargava. He gave this divine bow to his great-souled son Jamadagni, father of Parashurama. The bow later came into the possession of Parashurama.