Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.057.
Section : Making Ghusl for Hajj.
According to the scholars, it is also preferable for one who is undertaking the pilgrimage or ‘umrah to perform ghusl. Zaid ibn Thabit related that he saw the Messenger of Allah, when he intended to perform the hajj, perform ghusl. (Related by ad-Daraqutni, al-Baihaqi and at-Tirmizhi, who called it hassan. As-Usaili regarded it as weak.)