Vaisesika Sūtra 3.2.19

सुखदुःखज्ञाननिष्पत्त्यविशेषादैकात्म्यम् ॥१९॥

sukha-duḥkha-jñāna-niṣpatti-aviśeṣāt aikātmyam ॥19॥

[सुख (sukha) - pleasure; दुःख (duḥkha) - pain; ज्ञान (jñāna) - knowledge; निष्पत्त्य (niṣpatti) - production / attainment / experience; अविशेषात् (aviśeṣāt) - due to non-distinction; ऐकात्म्यम् (aikātmyam) - oneness;]

Due to non-distinction in the attainment of pleasure, pain, and knowledge, there is oneness.