Sahih Muslim Book 4, Hadith Number 1797.
Chapter : Reciting the Qur’an leisurely and distinctively and abstaining from reciting it too hurriedly, and permission to recite two suras of more than two in a Rak’ah.
Abu Ishaq reported: I saw a man asking Aswad b. Yazid who taught the Qur’an in the mosque: How do you recite the verse (fahal min muddakir) whether (the word muddakir) Is with (d) or (dh)? He (Aswad) said: It was with (d). I heard Abdullah b. Mas’ud saying that he had heard the Messenger of Allah (may peace be upon him) reciting (muddakir) with (d).