The Messenger of Allah ﷺ said, "Whoever does not thank people has not thanked Allah."
قال رسول الله ﷺ: لا يشكر الله من لا يشكر الناس.
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Ahmad (21838) narrated it from Waki, from Sufyan, from Salm bin Abdul-Rahman, from Ziyad bin Kulayb, from Ashath bin Qays, who mentioned it. Its narrators are trustworthy, and the apparent soundness of its chain notwithstanding, there is a break in it, for Ziyad bin Kulayb did not meet Ashath bin Qays, since Ashath died in the year 40 AH while Ziyad bin Kulayb died in the year 119 AH. Between their two deaths lies about eighty years, and I have not been able to determine his age at the time of his death.Abdul-Rahman bin Adi followed him in narrating from Ashath bin Qays. Bayhaqi related it in Shuab al-Iman (8699), and in his Sunan (6/182), by way of Muhammad bin Talha, from Ubaydullah bin Sharik, from Abdul-Rahman bin Adi, and he mentioned it. No one other than Abdullah bin Sharik narrated from Abdul-Rahman bin Adi. By these two chains, the hadith rises to the level of hasan.Bearing a similar meaning is what has been narrated from Abu Saeed Khudri, that the Messenger of Allah ﷺ said, "Whoever does not thank people has not thanked Allah." Tirmidhi (1955) and Ahmad (11280) both narrated it from the hadith of Ibn Abu Layla, from Atiya, from Abu Saeed Khudri, who mentioned it. Ibn Abu Layla is Muhammad bin Abdul-Rahman, whose memory was poor, but he was followed by Mutarrif bin Tareef, from Atiya, as found in Tabarani's al-Awsat (3606). Its defect lies in Atiya, who is Atiya bin Saad bin Junada, a man who errs frequently and requires corroboration, which I have not found.Bearing a similar meaning as well is what has been narrated from Usama bin Zayd, who said, the Messenger of Allah ﷺ said, "The most thankful of people to Allah is the one most thankful to people." Tabarani narrated it in al-Kabir (1/135), and Bayhaqi in Shuab al-Iman (8697), and in it is Abdul-Muneim bin Naeem al-Aswari, Abu Saeed al-Misri, the water-carrier, who is daif according to the majority of scholars. Al-Hafiz said in al-Taqreeb, "Discarded," and Haythami faulted the hadith on this account in al-Majma (8/181).The meaning of the hadith is that whoever is accustomed to not thanking people for their kindness while they are present before him, how then could he thank Allah for His favor when he does not see Allah ?