The people used to fast on ‘ashura (the tenth 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 Apostle said, “Whoever wishes to fast (on the day of ‘ashura’) may do so; and whoever wishes to leave it can do so.”
“The Prophet, may Allah bless him and grant him peace, came to Madina and saw the Jews fasting the day of ‘Ashura’ and said, ‘What is this?’ They said, ‘This is a holy day. This is the day when Allah rescued the Children of Israel from their enemy and so Musa fasted it.’ He said, ‘We have more right to Musa than you.’ So he fasted it and commanded that it be fasted.'”
The Messenger of Allah (may Allah bless him and grant him peace) said:
“The best of fasts after the month of Ramadan are in the Month of Allah, which you call Muharram. And the best of prayer after the obligatory prayer is the night prayer.”