It's listed as one of the requirements to apply for a tourist visa on the evisa website.
https://www.thaievisa.go.th/tourist-visa
Required documents
1. Passport or travel document with validity not less than 6 months.
2. Photograph of the applicant, taken within the past six months.
3. Evidence of travel from Thailand (air ticket paid in full).
4. Evidence of adequate finance (20,000 Baht per person and 40,000 Baht per family).