Declining Generation
Sources

Declining (reduced ability) of generations - Many places in the Talmud - here are a few: Eiruvin 53a, Shabbat 112b, Makot 24a

Oral Torah was forbidden to write - Talmud, Gittin 60b

Prayers were only a few pages long - Talmud Brachot 11b, Tamid 5:1

Learning Torah only while standing up - Talmud, Berachot 21a

The Falling Suka of David - Amos 9:11 and commentries on the verse.

Emphasis on the Study of Torah in the first generations and Good Deeds in the last generations - Tanya Pt. 4 Ch. 9

The Study of Torah is tantamount to everything - Pe'a 1:1

The Jewish People will only be redeemed through Charity - Talmud, Shabbat 139a. Maimonides, Laws of Presents for the Poor, Ch. 10:1

Heels / Footsteps of Moshiach - Psalms 89:52

Last generations like heel in comparison with first generations - Tanya Pt.1 Ch. 2

Spiritual insensitivity as means of handling Deep Evil - See the General Introduction to the Unpleasant Signs of Moshiach

Can reach much higher with "Deep Torah" than otherwise with one's own IQ -