That is found in Isa 41:15. Several years ago a friend shared with me what this passage had come to mean to her. Dixie was a very independant gal and a life-long Christian Scientist. She often amazed me with her deep insights.
Threshing was the process of separating the grain from the straw, what was of value from what was useless. Done on a threshing floor area, preferably where there was a breeze, the sheaves were loosened. The grain could then be tossed high in the air with a shovel or fork, the chaff would blow away and the clean grain fall to the ground.
The Bible passager could be interpreted as God's promise to provide us with the insights or materials we need to winnow out wrong thoughts or ideas. But Dixie had a second way to use this. She saw it as God's promise to make THEE a new sharp threshing instrument. We ourselves become the means of doing away with illness, lack, unhappiness, discord, etc..
The passage continues: Thou shalt thresh the mountains, and beat them small, and shalt make the hills as chaff. We all have times when problems loom as big as mountains, and as seemingly immovable. Too big to climb over, not enough strenght to even try, set in stone. But here is the promise that we can have that mustard seed amount of faith that allows us to toss that mountain right into the sea. It reminds me of another promise in the Bible found one chapter earlier, "Every valley shall be exalted, and every mountain and hill shall be made low: the crooked shall be made straight, and the rough places plain". (Isa 40:2) I pray with this one all the time.
So it is comforting to know that WE are the threshing instrument, backed by the power of divine Love to handle whatever comes up. We can separate the good from the bad, the grain from the chaff, and know that useless stuff will just blow away on the wind.