What is the evidence for stipulating the truthfulness in the Quran and the Prophetic Sunnah?
Allah said ( interpretation of the meaning ):
{ Alif, Lam, Meem Do the people think that they will be left to say, "We believe" and they will not be tried? But We have certainly tried those before them, and Allah will surely make evident those who are truthful, and He will surely make evident the liars. }
Quran ( 29 : 1-3 )
in addition to many other verses.
If anyone testifies sincerely that there is no true god except Allah, and Muhammad is His slave and Messenger, truly from his heart, Allah will safeguard him from Hell."
Narrated Talha bin 'Ubaidullah: A bedouin with unkempt hair came to Allah's Messenger (may the peace and blessings of Allah be upon him) and said, "O Allah's Messenger! Inform me what Allah has made compulsory for me as regards the prayers.Thus, Allah's Messenger (may the peace and blessings of Allah be upon him) informed him about all the rules (i.e. fundamentals) of Islam. The bedouin then said, "By Him Who has honored you, I will neither perform any Nawafil nor will I decrease what Allah has enjoined on me. Allah's Messenger (may the peace and blessings of Allah be upon him) said, "If he is saying the truth, he will succeed (or he will be granted Paradise).