The Ruling on One praying a Non-Obligatory Prayer leading One Who is performing an Obligatory Prayer

Question :

What is the ruling on one praying a non-obligatory prayer leading one who is performing an obligatory prayer? That is, if a person was performing the non- obligatory prayer then another person came and, thinking that he was performing the obligatory prayer, prayed with him; then someone else explained to him that what he did was not correct so he repeated his prayer. Was his first or second prayer acceptable?

Answer:

It is permitted for the one who is performing an obligatory prayer to be led by one who is performing a non-obligatory prayer, according to the story of Mu'ath, may Allah be pleased with him, who used to pray the 'Isha' prayer with the Prophet and then return to his people and lead them in the same prayer.

And the Prophet led a number of his Companions, may Allah be pleased with them, in the fear prayer consisting of two Rak'ahs, then he made the Taslim with them and prayed two Rak'ahs with another group, then he made the Taslim with them.
His second prayer was a non-obligatory prayer.
Source:
The Permanent Committed
Fatawa Islamiyah, Vol. 2 Page 179

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