Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 12
Sanskrit:
यदा कर्मविपाकेषु लोकेषु निरयात्मसु ।विरागो जायते सम्यङ् न्यस्ताग्नि: प्रव्रजेत्तत: ॥ १२ ॥
ITRANS:
yadā karma-vipākeṣulokeṣu nirayātmasuvirāgo jāyate samyaṅnyastāgniḥ pravrajet tataḥ
Translation:
If the vānaprastha, understanding that even promotion to Brahmaloka is a miserable situation, develops complete detachment from all possible results of fruitive activities, then he may take the sannyāsa order of life.