Valmiki Ramayana Bala Kanda Sarga 20 Shloka 18

महाबल: he, possessing great strength, यदा when, स्वयम् on his own, यज्ञस्य for sacrifice, विघ्नकर्ता न does not create impediments, तेन by, सञ्चोदितौ prompted, मारीचश्च Maricha, सुबाहुश्च Subahu, द्वौ राक्षसौ two rakshasas, यज्ञविघ्नम् obsctacles of sacrifice, करिष्यत: cause.

Although endowed with great strength, Ravana, by himself, never created impediments to the sacrifice. But two rakshasas by name Maricha and Subahu are incited by him to cause obstacles".