وَإِنْ كَانَ مُنْكِرًا، وَجَبَ تَوْكِيدُهُ بِمُؤَكِّدٍ أَوْ مُؤَكِّدَيْنِ أَوْ أَكْثَرَ حَسَبَ دَرَجَةِ الْإِنْكَارِ
If the listener denies the report, it is reinforced by one, two or more emphatic elements according to the degree of denial.
This is the inkārī type of report. The source's “wajaba” expresses a requirement of rhetorical suitability: the form should be fit to confront the hearer's denial.
Increasing the emphasis is not simply speaking louder. It involves selecting linguistic constructions that intensify the asserted proposition.