Hunaydah ibn Khaalid narrated upon the authority of his wife who said, “Some of the wives of the Prophet (salallaahu ‘alaihiwasallam) told me that the Prophet (salallaahu ‘alaihiwasallam) used to fast the Day of ‘Aashoorah, the first nine days of Dhul-Hijjah, and three days out of every month*.” (Imam Ahmad, An-Nasaaee)
*- 13th, 14th, and 15th every month.
Should we fast the first nine days of Dhul Hijjah or just on the 9th?
Fasting any of the nine days or all of them is fine. In terms of priority/preference: all nine days, otherwise at very least the day of 'Arafah and any or as many of those days as possible. The hadeeth is general in nature about good deeds and fasting is one of the best of those deeds. In addition, there are the reports indicating that the Prophet (SAW) fasted all those days (but there's a report in Saheeh Muslim where Aishah says she never saw him fast those days... there's a long explanation to this) and so one can safely say that fasting all the nine days is mustahabb Inshaa Allaah. And Allaah knows best
