Tosefet Bracha - Tazria

“He shall cry out ‘impure! Impure!’” (13; 48)
The gemara in Bava Kama (92a) says: Rava asked Rabba bar Meri, ‘where is the source in the Torah for this statement that people say ‘one bad thing follows another’?’ He replied, ‘the verse states “He shall cry out ‘impure! Impure!”.
The meaning of this is that it is not enough that a person with tzora’at has this affliction on his skin, but the Torah compounded his troubles by compelling him to inform everyone that he is impure, so that they will keep away from him.
In the continuation of the Talmud there they ask many similar questions – what is the source in the Torah for a certain popular statement. The question itself ‘what is the Torah source’ implies that there should be a source in the Torah for it, and the difficulty is only in extracting it from the text. This is strange. Why should we assume that there is a source in the Torah for each of these statements perhaps they are just expressions that people say.
The answer must be based on the concept in the midrashim, that any statement which is accepted or used by everyone, must certainly be true. For example the midrash says ‘the voice of the multitude is like the voice of G-d’. Given that, we can be certain that it is contained within the Torah, since it is the source of all truth. Therefore they can ask for a source and expect an answer.
Look also at Ta’anit 9a