Muwatta Malik, Book 8, Hadith 28 (USC-MSA)

Yahya related to me from Malik from Da'ud ibn al-Husayn that Ibn Yarbu al-Makhzumi said, "I heard Zayd ibn Thabit say, 'The middle prayer is the prayer of dhuhr.' "

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ دَاوُدَ بْنِ الْحُصَيْنِ، عَنِ ابْنِ يَرْبُوعٍ الْمَخْزُومِيِّ، أَنَّهُ قَالَ سَمِعْتُ زَيْدَ بْنَ ثَابِتٍ، يَقُولُ الصَّلاَةُ الْوُسْطَى صَلاَةُ الظُّهْرِ ‏.‏

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also referenced under: Book 8, Hadith 317 (In-Book)