Srimad Bhagavatam Canto 11, Chapter 18, Text 12

SB 11.18.12

yada karma-vipakesu
 lokesu nirayatmasu
virago jayate samyan
 nyastagnih pravrajet tatah
 
Translation: 
 
If the vanaprastha, 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 sannyasa order of life.
Srimad Bhagavatam Canto 11, Chapter 18, Text 11
Srimad Bhagavatam Canto 11, Chapter 18, Text 13