Sunan an Nasai 2408
The Book of Fasting
أَخْبَرَنَا زَكَرِيَّا بْنُ يَحْيَى، قَالَ حَدَّثَنَا عَبْدُ الأَعْلَى، قَالَ حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ ثَابِتٍ، عَنْ أَبِي عُثْمَانَ، أَنَّ أَبَا هُرَيْرَةَ، قَالَ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " شَهْرُ الصَّبْرِ وَثَلاَثَةُ أَيَّامٍ مِنْ كُلِّ شَهْرٍ صَوْمُ الدَّهْرِ " .
It was narrated that Abu Hurairah said:" I heard the Messenger of Allah say: 'The month of patience ( Ramadan) and three days of each month is fasting for a lifetime."' (Sahih)
Grading: Abu Ghuddah: Sahih · Al-Albani: Sahih · Zubair Ali Zai: Isnaad Sahih
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.