If a fasting person intentionally ascribes something false to Allah the Exalted, the Most Noble Prophet (Ṣ), or the Infallible Imams (ʿA) – whether he does this verbally, in writing, or by making a sign and suchlike – and even if he immediately says, ‘I have lied’ or he repents, then based on obligatory precaution, his fast is invalid.
The same applies, based on recommended precaution, to ascribing something false to Her Eminence [Fāṭimah] al-Zahrāʾ (ʿA) and the other Prophets and their successors.