More than a month after a hold-up at Rite Aid on Washington Street, police have a suspect. On May 26 at 1:15 p.m., a man in the store told an employee he had a gun and wanted money. The employee reported she had seen what looked like a silver handgun sticking out of a black backpack, and the robber took $350 in cash, police said. Last month, Richard Stone, 39, of Stoughton was arrested by Stoughton police in connection with a June 12 armed robbery and was later sent to the Dedham House of Corrections, according to Norfolk County Sheriff spokesman David Weber. While in custody, Stone asked to speak with a Canton officer and admitted holding up the Rite Aid, according to Canton Deputy Police Chief Helena Findlen. Police issued the summons to Stone on June 27, and charged him with armed robbery and assault with a dangerous weapon.