Hadith 8:2961
Abu Bakr narrated to us, he said: Ali bin Hashim narrated to us, from Isma'il, from al-Hasan, he said: If the Muhrim (pilgrim in the state of Ihram) wishes, he may perform ghusl (ritual bath), and if he wishes, he may not perform ghusl.
حدثنا أبو بكر قال نا علي بن هاشم عن إسماعيل عن الحسن قال إن شاء المحرم اغتسل وإن شاء لم يغتسل