The Messenger of Allah ﷺ said, "Whoever does not thank people has not thanked Allah."He said, this is a hasan sahih hadith.
حدثنا أحمد بن محمد أخبرنا عبد الله بن المبارك حدثنا الربيع بن مسلم حدثنا محمد بن زياد
قال رسول الله ﷺ من لا يشكر الناس لا يشكر الله قال هذا حديث حسن صحيح
[Its chain of transmission is sahih]. Abu Dawud also reported it in the Book of Manners, chapter on gratitude for kindness, hadith no. 4811, through Rabi bin Muslim with the same chain, and Ibn Hibban graded it sahih, hadith no. 2070.