And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Matthew 17:20
And the Lord said, If ye had faith as a grain of mustard seed, ye might say unto this sycamine tree, Be thou plucked up by the root, and be thou planted in the sea; and it should obey you. Luke 17:6
These are the only two scriptures in the Bible (that I know of) that talk about mustard seed faith. It seems that what Jesus is saying is if you have small faith (the mustard seed size), you can speak to impossible situations and they will be changed in your favor.
Now, I don’t profess to be a Bible scholar but when I read the Bible, I trust God to lead me to understanding by His Holy Spirit. In this video, I share what it is that I believe mustard seed faith is and how it pertains to life: