I was praying the rawatib (regular voluntary prayers) of Fajr in the mosque, and a man completed the prayer with me. Is it permissible for me to convert it into an obligatory prayer?

Question:
Honorable Shaykh, may Allah grant you success. He says: I was praying the rawatib of Fajr in the mosque, and a man came and stood beside me and completed the prayer with me. Is it permissible for me to convert it into an obligatory prayer?

Answer:
No, a voluntary prayer is not converted into an obligatory one. The reverse is the case: an obligatory prayer is converted into a voluntary one. As for converting a voluntary prayer into an obligatory one, no, this is not permissible and it is not valid.