Yoga Sutras 2.34
Patanjali, explained by Swami Vivekananda
वितर्का हिंसादयः कृतकारितानुमोदिता लोभक्रोधमोहपूर्वका मृदुमध्याधिमात्रा दुःखाज्ञानानन्तफला इति प्रतिपक्षभावनम् ॥
vitarkā hiṃsādayaḥ kṛtakāritānumoditā lobhakrodhamohapūrvakā mṛdumadhyādhimātrā duḥkhājñānānantaphalā iti pratipakṣabhāvanam
The obstructions to Yoga are killing, falsehood, etc., whether committed, caused, or approved; either through avarice, or anger, or ignorance; whether slight, middling, or great; and they result in infinite ignorance and misery. This is (the method of) thinking the contrary.
In plain words: Harmful acts like violence and lying hold us back, whether we do them, make others do them, or just approve of them. They can come from greed, anger, or ignorance, be small or large, and they always lead to more suffering and confusion. Remembering this is how you think the opposite thought.
Swami Vivekananda explains
If I tell a lie, or cause another to tell one, or approve of another doing so, it is equally sinful. If it is a very mild lie, still it is a lie. Every vicious thought will rebound, every thought of hatred which you may have thought, in a cave even, is stored up, and will one day come back to you with tremendous power in the form of some misery here. If you project hatred and jealousy, they will rebound on you with compound interest. No power can avert them; when once you have put them in motion, you will have to bear them. Remembering this will prevent you from doing wicked things.