משנה אויף ויקרא 2:9

[If one said,] “I take upon myself to bring [pieces of] wood,” he must bring not less than two logs. “Frankincense,” he must bring not less than a handful. There are five cases of [not less than] a handful: One who says, “I take upon myself to bring frankincense,” he must bring not less than a handful. One who voluntarily offered a minhah must bring a handful of frankincense with it. One who offered up the handful outside [the Temp] is liable. The two dishes [of frankincense] require two handfuls.