[Kanegasi]

If you don't want an entire addon just for a spell id, paste the following at https://addon.bool.no

local function spellid(self,unit,index,filter)
    local _,id
    if unit then
        _,_,_,_,_,_,_,_,_,id=UnitAura(unit,index,filter)
    else
        _,id=self:GetSpell()
    end
    if id then self:AddLine("ID: "..id) end
    self:Show()
end
hooksecurefunc(GameTooltip,"SetUnitAura",spellid)
hooksecurefunc(GameTooltip,"SetUnitBuff",spellid)
hooksecurefunc(GameTooltip,"SetUnitDebuff",spellid)
GameTooltip:HookScript("OnTooltipSetSpell",spellid)

If you want the line a certain color, change the AddLine part to this:

if id then self:AddLine("|cff789ABCID: "..id.."|r") end

Replace 789ABC with an appropriate hex value.