مَنْ لَمْ يَشْكُرِ النَّاسَ، لَمْ يَشْكُرِ اللَّهَ
Whoever does not thank people has not thanked Allah.
Source: أحمد والترمذي
لَمْ negates an action and puts the following present-tense verb in the jussive form, often corresponding to “did not.” The verb يَشْكُرْ comes from the root ش ك ر, “to thank.” النَّاسَ is the object—those people whose assistance should be acknowledged. The clauses connect failure to thank people with a deficient spirit of gratitude.
Allah's blessings often reach us through the work and care of other people. Expressing appropriate thanks, recognizing their contribution, and not taking help for granted are part of learning to be thankful. Gratitude is a sustained attitude, not only a memorized phrase.