Manava Dharmashastra 4.258

4.258

kṛte sarvakarmasaṃnyāsa idaṃ tasya viśeṣataḥ kartavyam |
ekākyasahāyaḥ sannavidyamānasambhāṣo'nākule vivikte nirjane rahasi cintayeddhyāyeddhitamātmanyupaniṣatsu yā brahmopāsanā vihitāstā abhyasyet |
taccintayā tadabhyāse paraṃ śreyo mokṣākhyamadhigacchati prāpnoti |

Medhātithi’s commentary (manubhāṣya):
The renunciation of all acts having been accomplished, the following is his special duty:—

‘Alone,’— without a companion,—having no one to talk to—‘in solitude’— in a place where there is no one, and which is free from all disturbance,—‘he shall meditate upon what is good for himself’; i.e., should practise that ‘contemplation of the Brahman’ which has been enjoined in the Upaniṣads.

By means of this practice and meditation, he obtains the highest good—called ‘Liberation.’—(258)