Muwatta Imam Malik Book of Hajj, Hadith 177

Malik was asked about whether a man could stand at Arafa, or at Muzdalifa, or stone the Jamras, or do say between Safa and Marwa if he was not in wudu, and he said, "Every practice in the hajj that a menstruating woman can take part in can be taken part in by a man who is not in wudu and there is nothing due from him for that. However, it is better for him to be in wudu for all those things, and he should not make a general practice of it."
Malik was asked whether a man who was riding should get down to do the standing at Arafa or if he could stand while mounted, and he said, "He can stand while mounted, unless he or his riding beast have an illness, in which case Allah is the one who most often accepts an excuse."