Is it obligatory to perform ablution for every voluntary prayer (such as Duha prayer) if a Muslim has already performed ablution for a previous obligatory prayer (such as Fajr prayer), or is tayammum sufficient, according to the Maliki school of thought?

If a Muslim performs ablution for the Fajr prayer and his ablution remains valid, he may pray the Duha prayer or other prayers with it, though it is commendable to renew the ablution. However, if the ablution is invalidated, tayammum is not prescribed for the Duha prayer or any other prayer as long as water is available and can be used without harm. The condition of ablution for prayer includes both obligatory and supererogatory prayers.