Ramayana
श्रूयते हि पुरावृत्तं वासवे सुमहात्मनि । ब्रह्महत्यावृतः शक्रो हयमेधेन पावितः ॥ 7-84-3 ॥
"For it is heard that in olden times, in the case of the great-souled Vasava, Shakra, who had been afflicted by the sin of brahmin-slaying, was purified through the horse-sacrifice."
english translation