Mahabharata 1.188.15

गुरोश्च वचनं प्राहुर्धर्मं धर्मज्ञसत्तमगुरूणां चैव सर्वेषां जनित्री परमो गुरुः

gurośca vacanaṃ prāhurdharmaṃ dharmajñasattamagurūṇāṃ caiva sarveṣāṃ janitrī paramo guruḥ

Our plain-language meaning of this verse is waiting for scholar review.