Reference: http://mufti.af.org.sa/ar/content/هل-التيمم-لصلاة-النافلة-يمكن-أن-يصلي-به-الإنسان-الفريضة؟
Question: Is it possible to pray an obligatory prayer with the [same] tayammum [which was done] for a voluntary prayer?
Response: The scholars say: If the time for the obligatory prayer has arrived, and he has made tayammum for the voluntary prayer, [then] he can pray [both] the obligatory and voluntary prayers [with the same tayammum]. If, however, he made tayammum before the time [of the obligatory prayer] arrived, then some of the scholars are of the opinion that he should renew his tayammum when the time [of the obligatory prayer] arrives.
[They say that] tayammum is [a] permissible [legislated ease], and is not [to be understood as that which constitutes the] removal [of the state of minor impurity]; meaning that it is permissible for the prayer – but it does not remove you from the state of [minor] impurity. However, that which is correct in shaa.-Allaah is that the ruling regarding tayammum [for purification] is the [same] ruling as [using] water [for purification].
So if you made tayammum to pray a voluntary prayer – before the time of Zhuhr arrived, then there is no harm in you praying the obligatory prayer [with the same tayammum when the time arrives]; because we say that the ruling regarding tayammum [for purification] is the [same] ruling as [using] water [for purification].
