Berachos 7b

Gemara: "R' Yochanan said in the name of R' Shimon ben Yochai: From the day that Hashem created his world, there was no one who thanked him - until Leah came and thanked him, as it is said [at the birth of Yehuda] 'This time I shall thank Hashem."
Rashi says: "Because Leah saw with Ruach HaKodesh that Ya'akov will have twelve Shevatim, and he had four wives. When she had her fourth son, she thanked him that her portion was more than what should have been coming to her."
It is very difficult to understand - Adam, Avraham, Mesushelach, Noach, etc. etc. all never thanked Hashem?!
I'd like to suggest something based in part on the aforementioned Rashi. Possibly this is how the world worked then: Noach (or Adam, or Yitzchok, etc.) would go out of their house in the morning, and bump into a pile of gold. Now, the first thing we would say is, "Boruch Hashem!" But, they were extremely in tune with their spiritual selves and with what Hashem expected from them. So the first thing Noach would say is, "What did I do to deserve this pile of gold? It must have been mitzvah so-and-so that I did yesterday." In which case the pile of gold was richly deserved! So there wasn't occasion to thank Hashem, because he deserved that pile of gold based on the laws of reward and punishment that Hashem created the world with. However, in the situation of Leah, She got more than she deserved, like Rashi says, "that her portion was more than what should have been coming to her." Therefore, she thanked Hashem.