Sayyiduna Anas bin Malik (may Allah be pleased with him) said that Sayyiduna Qays bin Sa'd (may Allah be pleased with him) was, in relation to the Prophet (peace and blessings be upon him), like the chief of police in relation to a governor, overseeing his affairs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16601