Muhammad proceeded towards Nejd at the head of 400 or 700 men, after he had mandated Abu Dhar - in the Ummayad version, the Ummayad chief who killed Abu Dhar is given this honor: Uthman bin Affan - to dispose the affairs of Madinah during his absence.
The most authentic opinion according to "Saifur Rahman al Mubararakpuri", however, is that Dhat Ar-Riqa' campaign took place after the fall of Khaibar (and not as part of the Invasion of Nejd).