The Ruling on praying more than One Obligatory Prayer with a Single Ablution
Question :
Is it permissible for us to pray two obligatory prayers with a single ablution, without intention?
Answer:
Yes, it is permissible for a person to pray the Zuhr prayer for example, after making Wudhu', then when the time comes for 'Asr prayer, if he is still in a state of purity, to pray that prayer with the ablution of Zuhr, even if he did not make the intention when he performed the ablution to pray two obligatory prayers. This is because the ablution that he made for Zuhr, removed any impurities from him; and if impurity is removed, it does not return without cause, which is any of the well known things which nullify Wudhu'.
Indeed, when a person performs Wudhu', without the intention of prayer, such as when he does so in order to remove impurity only, he may pray as much as he wishes of the obligatory and the non-obligatory prayers, until his ablution is invalidated.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 225-226