assalamu alikum you have quoted the fatwa of Imaam maalik in Fatwa No : 25513 that Imaam Maalik was reported to have said that whoever invents a new thing in religion and claims it to be good is in fact accusing Muhammad , of betraying the Message (of Islam). He mentioned as evidence of this the Saying of Allaah the Almighty (that means): {This day I have perfected for you your religion and completed My favor upon you and have approved for you Islam as religion.} [Quran 5:3]. Is this authentic according to scholars? because Shatabi did not mentioned any chain, ibn hazam quoted the chain in his alAhkaam but sufis say that it is not authentic because Abdul Malik bin Habeeb is weak even according to Ibn Hazam and others see meezan al aitedaal. if the chain is weak thn why the scholars of islamweb and islamqa and many others quote this in evidence against innovation?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The late scholars used to mention the statements of the scholars of the Salaf (righteous predecessors) which were conveyed from them by trustworthy scholars in the past, among which is this common statement from Imaam Maalik which was reported by Imaam Ash-Shaatibi as you mentioned and also by Ibn Ash-Shaat in his book Anwaar Al-Burooq. If this statmeent is confirmed to be Dha’eef [weak] from Imaam Maalik, then this does not prove that there is evidence to support innovations, as innovations in religion are rejected by confirmed religious texts prior to this statement and there is evidence that innovations are misguidance for the general saying of the Prophet reported by Jaabir and Al-'Irbaadh: “…and every innovation is a misguidance….” [Muslim] This Hadeeth is also reported by Ibn ‘Abdul Barr with an authentic chain of narrators as stated by Ash-Shawkaani who added: “…And beware of the newly invented matters (in religion) as every newly invented matter is an innovation and every innovation is a misguidance.”

This generality in the saying of the Prophet “every innovation is a misguidance” indicates the baselessness of the statements of those who said: “There is a good innovation in Islam” and Ibn 'Umar said: “Every innovation is a misguidance even if people consider it as a good matter.” [Muhammad ibn Nasr Al-Marwazi, Al-Laalakaa’i and Ibn Battah; it was also classified as Saheeh (sound) by Al-Albaani]

Moreover, Ash-Shawkaani said in Al-Qawl Al-Mufeed: “Since Allaah had completed His religion before He took away the soul of His Prophet, then how come the people invented this opinion after Allaah had completed His religion? If it is from religion according to their belief, then the religion was not complete except with their opinion, and this is denying the Quran, and if it was not from religion, then why do they preoccupy themselves with something that is not from religion?”

Allaah Knows best.