Imam Ahmad : 8034
حَدَّثَنَا أَبُو النَّضْرِ حَدَّثَنَا شَيْبَانُ عَنْ عَاصِمٍ عَنِ الْأَسْوَدِ بْنِ هِلَالٍ عَنْ أَبِي هُرَيْرَةَ قَالَ أَمَرَنِي رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِثَلَاثٍ بِنَوْمٍ عَلَى وِتْرٍ وَالْغُسْلِ يَوْمَ الْجُمُعَةِ وَصَوْمِ ثَلَاثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ
Narrated to us [Abu An Nadhr] Narrated to us [Syaiban] from ['Ashim] from [Al Aswad bin Hilal] from [Abu Hurairah], he said: Rasulullah peace and blessings be upon him ordered me three things: witir before going to bed, bathing on Fridays and fasting for three days every month.