Ramayana
स मधुर्वीर्यसम्पन्नो धर्मे च सुसमाहितः । बहुवर्षसहस्राणि रुद्रप्रीत्या ऽकरोत्तपः ॥ 7-61-5 ॥
"That Madhu, endowed with valour and well-settled in righteousness, performed austerities for many thousands of years to please Rudra."
english translation