Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
suyuti:17125a –
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السيوطي:١٧١٢٥a –

" لَئِنْ بَقيتُ أمَرْتُ بِصِيَامِ يَوْم قَبْلَهُ أوْ يَوْمٍ بَعْدَهُ يَوْمُ عَاشُوراءَ".  

[هب] البيهقى في شعب الإيمان عن داود بن علي عن أبيه عن جده

See similar narrations below:

Collected by Bayhaqī
bayhaqi:8405 – Abū al-Ḥusayn b. al-Faḍl al-Qaṭṭān > ʿAbdullāh b. Jaʿfar > Yaʿqūb b. Sufyān > al-Ḥumaydī > Sufyān > Ibn Abū Laylá > Dāwud b. ʿAlī from his father from his grandfather

[Machine] The Messenger of Allah ﷺ said: "If I am spared, I will definitely order the observance of fasting on a day before or after the day of Ashura."  

البيهقي:٨٤٠٥ – وَأَخْبَرَنَا أَبُو الْحُسَيْنِ بْنُ الْفَضْلِ الْقَطَّانُ أنبأ عَبْدُ اللهِ بْنُ جَعْفَرٍ ثنا يَعْقُوبُ بْنُ سُفْيَانَ حَدَّثَنِي الْحُمَيْدِيُّ ثنا سُفْيَانُ عَنِ ابْنِ أَبِي لَيْلَى عَنْ دَاوُدَ بْنِ عَلِيٍّ عَنْ أَبِيهِ عَنْ جَدِّهِ

أَنَّ رَسُولَ اللهِ ﷺ قَالَ لَئِنْ بُقِّيتُ لَآمُرَنَّ بِصِيَامِ يَوْمٍ قَبْلَهُ أَوْ يَوْمٍ بَعْدَهُ يَوْمِ عَاشُورَاءَ