Sources on the lifespan of Dhamma Everyone is saying that the Dhamma will last 5000 years after the parinibbana. That it will last in Sri Lanka. So far I haven't found any authentic sources even in Tripitaka saying that 

Are there any sources if so please do provide. If not, is this some fact created by people in Sri Lanka back in the days ?
Please see the essay "The Coming Buddha, Ariya Metteyya" by Ven. Sayagyi U Chit Tin:


  During the period from the time of Buddha Gotama to the minimum life
  span, the Buddha's Dispensation (Buddha-sasana) will disappear. When
  the Buddha agreed to create the Bhikkhuni Sangha, he told Ven. Ananda
  that the Sasana would last only half as long because of this. Instead
  of lasting one thousand years, it would last five hundred years. The
  commentary on the Abhidhamma text, Dhammasangani, says that when the
  First Buddhist Council convened by Ven. Maha-Kassapa rehearsed the
  Pali Canon, this made it possible for the Sasana to endure for five
  thousand years.[48]
  
  The commentaries on the Vinaya Pitaka[49] and the Anguttara-nikaya[50]
  say that the eight important rules which the Buddha gave to the
  Bhikkhuni Sangha will make his Teachings last for five thousand years
  rather than five hundred. There will be one thousand years for Arahats
  who attain analytical insight, one thousand years for Arahats without
  those attainments, one thousand years for Non-returners, one thousand
  years for Once-returners, and one thousand years for Stream-winners.
  After these five thousand years of penetration of the true Doctrine
  (pativedha-sadhamma),[51] the accomplishment in the texts
  (pariyatti-dhamma) will remain. After the accomplishment in the texts
  disappears, the signs (linga) will continue for a long time.
  
  In the commentary to the Theragatha[52] the Sasana is said to consist
  of five periods: (1) the age of deliverance (vimutti-yuga), (2) the
  age of concentration (samadhi-yuga), (3) the age of morality
  (sila-yuga), (4) the age of learning [the texts] (suta-yuga), and (5)
  the age of generosity (dana-yuga). Ven. Dhammapala says, concerning the
  disappearance of learning, "In a region where there is no purity of
  morality, accomplishment (in the texts) remains through taking up
  great learning, through the desire to acquire, etc. But when
  accomplishment in the summary [i.e., the Patimokkha] is completely
  ended, it disappears. From that time on, only the mere sign (linga)
  remains. Then, having accumulated riches in various ways, they give
  away gifts (dana); this, truly, is the last right practice. Then, [the
  period starting] after the disappearance of learning is the last time
  (pacchima-kala). Others say that it is from the time of the
  disappearance of morality." According to the tradition in Burma, the
  Sasana will last five thousand years. The five periods will occur
  twice. The first half of the Sasana has just passed, with each of the
  five periods lasting five hundred years. We are now in the second
  half, when these periods will be repeated, each lasting for another
  five hundred years.




Footnotes:


  [48] As 27, see Expos. 35.
  [49] Sp 1291.
  [50] Mp IV 136f.


List of Abbreviations:

As =   Atthasalini (trans., Expos.).
       BN 630/862 = Bibliotheque nationale, Pali mss 
       630 and 862, commentary on Anag. We thank Mme J. 
       Filliozat for making available a copy of her 
       transcription of this text.
Expos.=The Expositor (As).
Sp =   Samantapasadika (commentary on the Vinaya Pitaka).
Mp =   Manorathapurani (commentary on the Anguttara Nikaya).