Abu Ishaq (may Allah have mercy on him) says that I saw a man asking Aswad bin Yazid—who was teaching people the Qur'an in the mosque—"Do you recite this letter «فَهَلْمِنْمُدَّكِرٍ» with a 'dal' or with a 'dhal'?" He replied: "With a 'dal'. I heard Sayyiduna Ibn Mas'ud (may Allah be pleased with him) say that I heard the Prophet (peace and blessings be upon him) recite this word with a 'dal'."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4401