Narrated Jarir bin ‘Abdullah رضي الله عنه :
The Prophet صل الله عليه و سلم said, “Allah will not be merciful to those who are not merciful to mankind.”
[Sahih Bukhari Volume 9, Book 93, Number 473]
Needs no commentary from me. Although, if anyone has commentary on it from professional sources, I would love to read it.