The ruling on a person praying an obligatory prayer following a person praying a voluntary prayer

Question:
A man arrived after the prayer had ended and walked behind a man who was praying, so he stood to his side and said, “May Allah send blessings and peace upon him,” while praying with him. So, is the condition for the validity of his prayer the imam’s words? It is not to be in the recitation.

Answer:
There is no harm in that, and according to the correct view, it is permissible for one who is praying an obligatory prayer to follow one who is praying a voluntary prayer, just as the Companions (may Allah be pleased with them) used to pray behind Mu’adh (may Allah be pleased with him), even though he had already prayed with the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) insisted on that. So, there is no harm in that. And reciting aloud is not obligatory; rather, it is a recommended practice (sunnah).