"After Da3sh [derogatory name for ISIS] transgressed all the restraints according to Shari'a and carried out its attacks on the populace of Muslims and on factions that had gone out to wage jihad against the Nusayri enemy and its prisons became filled with detainees and on its head the commander of Jabhat al-Nusra in Raqqa Abu Sa'ad al-Hadrami and the despairing attempts of the Shari'a officials in Jabhat al-Nusra who tried to stop the bloodshed and restore genuine relations of brothers between the factions, Jabhat al-Nusra has undertaken a military operation against Da3sh in response to its transgressions and restore the oppressed to their families; and the name given for this operation is "Freeing the Distressed from the Prisons of the Criminal" (freeing of prisoners) in Raqqa."
NB: It is claimed that this statement is fake. Possible. So caveat lector.
Further note: In light of the confusion as to the authenticity of this statement, some analysis and context are necessary. To begin with, it should be noted that when Jabhat al-Nusra first announced its return to Raqqa in September, multiple FSA banner remnants pledged allegiance either to Nusra or ISIS. These FSA banner remnants, as Shami Witness, myself and others have noted, were autonomous and never quite integrated into the Nusra hierarchy as elsewhere in the country. Thus, this statement does not have any approval from Jowlani or others in the JN central hierarchy, and is to be attributed to those FSA banner remnants who pledged allegiance to Nusra and use the Nusra banner but were not properly incorporated into the organization. There is of course no way Jowlani would declare war on ISIS.
Update: Note that Jabhat al-Nusra's central leadership officially denies clashes in Raqqa; and strictly speaking, that is correct in lieu of the further note above.