Vana Parva 278.17

Equipped with his bow, quiver and sword and with his fingers encased in the skin of Godha, Rama ran after that deer as Rudra followed the stellar deer (i.e. Prajapati, who in the guise of a deer followed his daughter, but Shiva cut off his head which became the constellation called Mrigashira i.e. the deerhead).