Prayer: Changing Intention from Ada to Qada
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 750
If there is ample time for an adāʾ prayer, one can, during the prayer, in the event that he remembers he has an outstanding qaḍāʾ prayer to perform, change his intention to the qaḍāʾ prayer. However, [for this to be valid,] it must be possible to change the intention to a qaḍāʾ prayer; for example, if he has started to perform the ẓuhr prayer, he can change his intention to a qaḍāʾ ṣubḥ prayer only if he has not yet reached the rukūʿ of the third rakʿah.