It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) taught me this verse in this way: « ﴿وَلَقَدْيَسَّرْنَاالْقُرْآنَلِلذِّكْرِفَهَلْمِنْمُدَّكِرٍ﴾ [القمر : 17]» A man asked: O Abu Abdur Rahman! Is the word "muddakir" with a dal or with a dhal? He replied: The Prophet (peace and blessings of Allah be upon him) taught me with a dal.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 3755