ॐ तत्पुरुषाय विद्महे महादेवाय धीमहि
तन्नो रुद्रः प्रचोदयात्॥
Om Tatpurushaya Vidmahe Mahadevaya Dhimahi
Tanno Rudrah Prachodayat॥
Om. Let me meditate on the great Purusha, Oh, greatest God, give me higher intellect, and let God Rudra illuminate my mind.
Also check Shiva Gayatri Mantra.