I think this comes from WHO, but isn't consistent with other information from WHO, so it's pretty debatable.
I believe the source is this[0], which says "Mental health conditions such as anxiety and depression are highly prevalent in all countries and communities, affecting people of all ages and income levels. They represent the second biggest reason for long-term disability, contributing to loss of healthy life."
However, elsewhere on their site[1], WHO lists the top 3 global causes of death and disability in 2021 as heart disease, COVID-19, and stroke.
[0] https://www.who.int/news/item/02-09-2025-over-a-billion-peop...
[1] https://www.who.int/data/gho/data/themes/theme-details/GHO/m...