'Mustard seed' in the Bible

It is like a grain of mustard seed, which a man took and sowed in his garden: and it grew, and waxed a great tree, and the fowls of the air made nests in the branches of it."

And the Lord said, "If ye had faith like a grain of mustard seed, and should say unto this sycamine tree, 'Pluck thyself up by the roots, and plant thyself in the sea': he should obey you.