How can one convince others that the jalabiya is the "qamis" (shirt/tunic) referred to in the saying of Umm Salama: "The most beloved garment to the Prophet, peace and blessings be upon him, was the qamis," and that it is a Sunnah of the Messenger of Allah, peace and blessings be upon him?

It is best for a Muslim to wear what the Prophet, peace and blessings be upon him, loved. However, there is no harm in wearing any modest, permissible clothing that does not involve extravagance or pride. The obligatory clothing is that which covers the 'awrah, and no specific attire is mandated. The qamis (tunic) was the most beloved garment to the Prophet, peace and blessings be upon him, because it provided more covering than the rida' (upper garment) and izar (lower garment), and not for its own sake. So, if the quality of modesty is found in other garments, they are equivalent to it. The basic principle regarding clothing is permissibility unless there is a specific religious prohibition, and ordinary matters like clothing are not among his legislative actions that must be followed.