Sahih Bukhari Volume 9, Book 90, Hadith 6960

وَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِذَا مَا رَبُّ النَّعَمِ لَمْ يُعْطِ حَقَّهَا، تُسَلَّطُ عَلَيْهِ يَوْمَ الْقِيَامَةِ، تَخْبِطُ وَجْهَهُ بِأَخْفَافِهَا ‏"‏‏.‏ وَقَالَ بَعْضُ النَّاسِ فِي رَجُلٍ لَهُ إِبِلٌ، فَخَافَ أَنْ تَجِبَ عَلَيْهِ الصَّدَقَةُ، فَبَاعَهَا بِإِبِلٍ مِثْلِهَا، أَوْ بِغَنَمٍ، أَوْ بِبَقَرٍ، أَوْ بِدَرَاهِمَ، فِرَارًا مِنَ الصَّدَقَةِ بِيَوْمٍ، احْتِيَالاً فَلاَ بَأْسَ عَلَيْهِ، وَهْوَ يَقُولُ إِنْ زَكَّى إِبِلَهُ قَبْلَ أَنْ يَحُولَ الْحَوْلُ بِيَوْمٍ أَوْ بِسَنَةٍ، جَازَتْ عَنْهُ‏.‏

Allah's Apostle added, "If the owner of camels does not pay their Zakat, then, on the Day of Resurrection those camels will come to him and will strike his face with their hooves." Some people said: Concerning a man who has camels, and is afraid that Zakat will be due so he sells those camels for similar camels or for sheep or cows or money one day before Zakat becomes due in order to avoid payment of their Zakat cunningly! "He has not to pay anything." The same scholar said, "If one pays Zakat of his camels one day or one year prior to the end of the year (by the end of which Zakat becomes due), his Zakat will be valid."