salaams Just because, a hadith is not specifically mention in the colections of Bukhari, Muslim or Malik - does not mean that they did not believe in the coming of Al Mahdi. Imam Thirmidhi, Imam Nasai and Imam Abu Dawood - were all students of Imam Bukhari and so was Imam Muslim. Imam Bukhari says about Imam Thirmidhi, that what I learnt from him is more than what he learnt from me. Imam Bukhari and Muslim, objective of writing there hadith was different from the others, neither of these muhaditheen wrote there books, to give us all the sahih hadith they know. In the collection Abu Dawood, he devotes a whole chapter to Imam Mahdi - If Imam Bukhari or muslim, did not believe in the coming of Al- Madhi then Imam Bukhari and also Imam Muslim, students (Nisai, Thirmidhi and Abu Dawood) - have not narrated these hadith in there collection!!! If I remember correctly - Imam Suyuti, writes a book on Imam Mahdhi, narrating 240 hadith on him - starting with the most authentic and then going into weak hadith - so the evidence from all the scholars of hadith are there are authentic hadith on him - it is best to believe in the coming of Imam Madhi as some scholars even consider it kuffar to deny his coming!!! Allah Knows Best