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 (ﷺ) 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 ‏"‏ ‏.‏