Sh. Muḥammad b. Ṣāliḥ al-ʿUthaymīn رَحِمَهُ اللهُ was asked about a person who normally fasts the ten days of Dhū al-Ḥijjah, and he wants to do Ḥajj – can he fast those ten days?

The Shaykh رَحِمَهُ اللهُ replied:

“Fasting the ten days of Dhū al-Ḥijjah is not obligatory, so if a person wishes he can fast them, and if he wishes he does not fast them, whether he travels for Ḥajj or remains in his country. This is because every fast that is optional, then the person has a choice about it: thus, if he is in his country and he wishes to fast then let him fast, and if he travels and sees fasting to be difficult then let him not fast, because it is not appropriate for one for whom fasting is difficult during travelling to fast – whether that be an obligatory or optional fast. However, in ʿArafah he is not to fast because the Messenger of Allāh ﷺ did not fast during the day of ʿArafah, and it has been narrated from him ﷺ that he forbade fasting (the day of) ʿArafah at ʿArafah.”

Majmūʿ Fatāwá wa Rasāʾil al-Shaykh Muḥammad b. Ṣāliḥ al-ʿUthaymīn, 20: 45, no. 402.

Note: The ḥadīth regarding the Prophet ﷺ forbidding the fasting of ʿArafah at ʿArafah (Abū Dāwūd (2440)) is graded weak by al-Albānī. See: Ḍaʿīf Abī Dāwūd (421) and al-Ḍaʿīfah (404).