(47) بَابُ قَوْلِ اللَّهِ تَعَالَى: {جَعَلَ اللَّهُ الْكَعْبَةَ الْبَيْتَ الْحَرَامَ قِيَامًا لِلنَّاسِ وَالشَّهْرَ الْحَرَامَ وَالْهَدْيَ وَالْقَلاَئِدَ ذَلِكَ لِتَعْلَمُوا أَنَّ اللَّهَ يَعْلَمُ مَا فِي السَّمَوَاتِ وَمَا فِي الأَرْضِ وَأَنَّ اللَّهَ بِكُلِّ شَيْءٍ عَلِيمٌ}
Narrated `Aisha:
The people used to fast on 'Ashura (the tenth day of the month of Muharram) before the fasting of Ramadan was made obligatory. And on that day the Ka`ba used to be covered with a cover. When Allah made the fasting of the month of Ramadan compulsory, Allah's Messenger (ﷺ) said, "Whoever wishes to fast (on the day of 'Ashura') may do so; and whoever wishes to leave it can do so."
Reference: Sahih al-Bukhari 1592
In-book reference: Book 25, Hadith 78
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 26, Hadith 662