Abu Dawud Book 25, Hadith Number 3983.

Narated By Abdullah ibn Mas’ud : The Prophet (PBUH) used to read the verse “Is there any that will receive admonition (muddakir)? ” that is with doubling of consonant [(dal)(d)].

Abu Dawud said: The word muddakir may be pronounced as mim (m) with short vowel u, (dal) (d) with a short vowel a and kaf (k) with a short vowel i.