Is the prayer of a voluntary worshipper valid behind one performing a mandatory prayer?

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There is no difference within the school regarding the validity of a voluntary worshipper praying behind one performing a mandatory prayer. We know of no difference of opinion among the scholars on this matter. This is indicated by the saying of the Prophet (peace be upon him): "Is there no man who will give charity regarding this, to pray with him?" and by the hadiths concerning the re-establishment of congregational prayer. Additionally, the obligation of the follower is fulfilled by the intention of the leader, as evidenced if one intends a required prayer but finds it is before its prescribed time.