Manava Dharmashastra 11.73

11.73

lakṣyaṃ śastrabhṛtāṃ vā syād viduṣām icchayātmanaḥ
prāsyed ātmānam agnau vā samiddhe trir avākśirāḥ //

For his purification the slayer of a Brahmana shall make a hut in the forest and dwell (in it) during twelve years, subsisting on alms and making the skull of a dead man his flag.