Whoever thanks people thanks Allaah
Is this a Hadeeth?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The Hadeeth that implies that meaning was reported in a different wording than that which our dear questioner mentioned.

It was reported that Abu Hurairah narrated a Marfoo‘ Hadeeth: “He who does not thank people (for their favors) has not thanked Allaah.” [Ahmad and Al-Bukhari in Al-’Adab Al-Mufrad, Abu Daawood, Ibn Hibaan, At-Tayaalisi] [Al-Albaani: Saheeh]

Moreover, Imaam Ahmad reported it as a Marfoo' Hadeeth with a similar wording  on the authority of  Al-Ash‘ath ibn Qays ; however, it is Munqati' Hadeeth.

Al-Khattaabi said: This Hadeeth can be explained in two ways:

First, a person who is accustomed to and whose nature is to be ungrateful to people and is used to not thanking them for their favors, would be ungrateful to his Lord The Almighty and would not thank Him for His Favors.

Second, Allaah The Almighty Does not Accept the thankfulness of the slave for His Favors if the slave does not thank the people for their favors.

This Hadeeth implies rebuking the one who does not thank people for their favors and urging everyone to be grateful for people’s favors.

Expressing thanks to people for their favors can be done by praising them and saying kind words to them, and also by supplicating Allaah The Almighty for them.

At-Tirmithi, An-Nasaa’i and Ibn Hibbaan reported that Usaamah ibn Zayd narrated a Marfoo' Hadeeth: “Whoever has a favour done for him and says to the one who did it, Jazaaka Allaahu khayran (may Allaah Reward you) has done his utmost to thank him.” [At-Tirmithi, Al-Albaani: Saheeh]

Expressing thanks can also be done by rewarding people in return for their favors, for it was narrated on the authority of Ibn Umar that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “Whoever does you a favor, reward him. However, if you do not find something to reward him with then supplicate for him until you feel that you have rewarded him.” [Ahmad, Al-Haakim, and other narrators, and Al-Arnaa’oot: Saheeh chain of narrators]

Allaah Knows best.