Bhagavad Gita · 8.25

धूमो रात्रिस्तथा कृष्णः षण्मासा दक्षिणायनम्।तत्र चान्द्रमसं ज्योतिर्योगी प्राप्य निवर्तते॥

dhūmo rātris tathā kṛṣhṇaḥ ṣhaṇ-māsā dakṣhiṇāyanam tatra chāndramasaṁ jyotir yogī prāpya nivartate

Departing during the influence of smoke, night, the dark fortnight, and the six months of the southern course of the sun, the yogi attains the lunar light and returns.

— Bhagavad Gita 8.25

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)