- Ani SuttaAni Sutta - have desire to learn the Dhamma as taught by the Buddha and not anyone else
- Anagata bhaya Sutta - teaching not practiced in earnest and procrastination
- Sugata Vinaya SuttaSugata Vinaya Sutta - monks become unapproachable and have wrong understanding
- Ovada Suttas - boastful about preaching ability, lack of faith, moral shame and fear, elders not setting a good example
- Kimbila Suttas - lack of mutual respect towards the teacher, Dhamma, Sangha, practice, each other, heedfulness and hospitality
- Saddhamma Patirupaka Sutta - counterfeit Dhamma emerging and unspiritual people lead to decline
- Appamada Sutta - heedlessness, indolence, excessive desires, unwise attention, lack of full awareness, evil friends, neglect of the wholesome lead to decline. Opposite preserves the Dhamma.
- Thiti Sutta / Parihana Sutta - Satipatthana not being practiced. Practicing preserves the Dhamma.
- Mahā,parinibbāna Sutta - being anxious to learn and practice Dhamma will result in the highest attainments [and preserve it]
- Vinaya Cullavagga - admission of women
- etc.
More on this see the references of: The Dharma-ending AgeThe Dharma-ending Age by Piya Tan
In addition the commentarial literature has the 5 antaradhāna which is mentioned in another answer. They are:
- adhigama-antaradhāna - attainments disappear, because the practice is polluted with miss conception.
- paṭipatti-antaradhāna - because attainments are disappear, the practice is neglected, hence it disappear
- pariyatti-antaradhāna - because practice are disappear, the learning is neglected, hence it disappear
- liṅga-antaradhāna - because learning are disappear, the monasticism (Sangha) is neglected, hence it disappear
- dhātu-antaradhāna - because reverence to wards the Dhātu disappear, the Dhātu is neglected, hence it disappear
- So ends the Sasana and beings the dark ages
