Seriousness of the Prohibition, as compared to that of Terumah: Yoma 83a
Penalty of Divine execution [mitah b'ydei shamayim] for eating tevel: Zevachim 11b
For Un-tithed Produce to become Leaven, although it is Already Prohibited under one Prohibition: Pesachim 35b-36a Punishment for violating this prohibition: Makkot 13a, 16b-17a
How much one must eat, in order to be liable: Makkot 13a, 17a
Defining "Tevel"
Produce from which all separations have not been done: Pesachim 35b
Status of Sprouts of Un-tithed Produce, and their sprouts, where the initial seeds do [not] survive: Pesachim 34a
Seeds planted in a pot without holes are Produce requiring Tithing Rabbinically: Shabbat 128a, Pesachim 35b