مُوَطَّأُ مَالِكٍ
Muwatta Malik 625
Zakat
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ سَأَلَ ابْنَ شِهَابٍ عَلَى أَىِّ وَجْهٍ كَانَ يَأْخُذُ عُمَرُ بْنُ الْخَطَّابِ مِنَ النَّبَطِ الْعُشْرَ فَقَالَ ابْنُ شِهَابٍ كَانَ ذَلِكَ يُؤْخَذُ مِنْهُمْ فِي الْجَاهِلِيَّةِ فَأَلْزَمَهُمْ ذَلِكَ عُمَرُ .
Yahya related to me from Malik that he had asked Ibn Shihab why Umar ibn al Khattab used to take a tenth from the Nabateans, and Ibn Shihab replied, "It used to be taken from them in the jahiliyya, and Umar imposed it on them."
Arabic/English book reference: Book 17, Hadith 49