मनु स्मृति - मनः पापः

परद्रव्येष्वभिध्यानं मनसानिष्टचिंतनम् |
वितथाभिनिवेशश्च त्रिविधं कर्म मानसम् ||

- मनु स्मृति

The following three are the sins committed by our mind (not our actions) - Longing for other people's treasures, thinking ill of others and having firm belief in the untruth.

- Manu Smriti