Bodhicaryāvatāra 5.1

śikṣāṃ rakṣitukāmena 

cittaṃ rakṣyaṃ prayatnataḥ  |

na śikṣā rakṣituṃ śakyā 

calaṃ <acc. to Tib. ayam> cittam arakṣatā ||

One desirous of guarding [one’s] precept

Should diligently guard [one’s] mind.

One who is not able to