Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
You are allowed to pray an obligatory Prayer behind a man who is praying a supererogatory Prayer and vice versa.
In his book, Al-Fiqh `ala al-Madhahib al-Arba`ah (Fiqh of the Four Schools of Thought), Sheikh `Abd Ar-Rahman Ag-Gazery states the following:
Performing prayer in congregation is a confirmed Sunnah. Ibn Umar (may Allah be pleased with him) quoted the Prophet (peace and blessings be upon him) as saying: “The reward of prayer performed in congregation outweighs that of prayer performed individually by twenty-seven degrees.” (Al-Bukhari and Muslim)
According to the four schools of fiqh, a person is allowed to pray behind an imam who is praying an obligatory Prayer while he is intending to pray an optional Prayer.
But, they differed regarding praying obligatory Prayers behind an imam who is praying optional Prayers. The Shafi`i school of Fiqh see no problem in this.
Almighty Allah knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.