The Messenger of Allah ﷺ said, "Allah does not show mercy to one who does not show mercy to people."
قال رسول الله ﷺ: لا يرحم الله من لا يرحم الناس.
No references are available.
Bukhari narrated it in Adab (6013) and in Tawhid (7376), and Muslim narrated it in Fadail (2319:66), both through Amash, from Zayd bin Wahb and Abu Dhubyan, from Jareer bin Abdullah, who said, then he mentioned it.