What is the expiation for intentionally lying about the Prophet, may Allah bless him and grant him peace, and is there repentance for that?

Lying about the Prophet (peace be upon him) is one of the gravest major sins; some even consider it disbelief (kufr). Sincere repentance for it is obligatory, involving remorse and a firm resolve never to return to it. Allah forgives all sins for those who repent and seek forgiveness, and the questioner's remorse and eagerness to expiate their sin are important steps towards repentance. The questioner should increase in righteous deeds and seeking forgiveness, and should inform those before whom the fabricated narration was told that it is not a prophetic hadith. If this is not possible, it does not prevent the acceptance of their repentance.