Is it permissible to name a son "Sumhammad" or must one be restricted to "Muhammad"?

There is no شرعًا (religiously) objectionable aspect in naming a newborn "Muhammad Sayyid." Some do this as an abbreviation for the word "Sayyid." However, it is preferable to keep his original name "Muhammad" so that he may attain the honor of being named after the Seal of the Prophets.