Your Loss Is Your Gain
The Gemara (Pesachim 87b) records a conversation that Hashem had with Hoshea HaNavi that is not mentioned at all in Tanach. Tosfos and Rashi both explain, however, how this conversation is alluded to in the verses.
Rosh Hashanah
The Talmudic teaching that a person's losses for the year are set on Rosh Hashanah, and how giving charity can fulfill that decree.
The Gemara (Pesachim 87b) records a conversation that Hashem had with Hoshea HaNavi that is not mentioned at all in Tanach. Tosfos and Rashi both explain, however, how this conversation is alluded to in the verses.