Sunan Abi Dawud, Hadith 3914

Abu Dawud said: Malik was asked about the meaning of his saying: There is no safar. He replied: The people of pre-Islamic Arabia used to make the month of safar lawful (for war). They made it lawful in one year and unlawful in another year. The Prophet said: There is no safar.

قَالَ أَبُو دَاوُدَ قُرِئَ عَلَى الْحَارِثِ بْنِ مِسْكِينٍ وَأَنَا شَاهِدٌ، أَخْبَرَكُمْ أَشْهَبُ، قَالَ سُئِلَ مَالِكٌ عَنْ قَوْلِهِ ‏"‏ لاَ صَفَرَ ‏"‏ ‏.‏ قَالَ إِنَّ أَهْلَ الْجَاهِلِيَّةِ كَانُوا يُحِلُّونَ صَفَرَ يُحِلُّونَهُ عَامًا وَيُحَرِّمُونَهُ عَامًا فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ لاَ صَفَرَ ‏"‏ ‏.‏

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also referenced under: Hadith 3904 (Hasan)