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 whyUmar ibn al Khattab used to take a tenth from the Nabateans, and IbnShihab replied, "It used to be taken from them in the jahiliyya, andUmar imposed it on them." Translation Not Available

Book reference: Muwatta Imam Malik Book 17 Hadith 625
Web reference: Muwatta Imam Malik Book 17 Hadith 49