Charles H. Spurgeon on Psalms 141:3

Set a watch, O LORD, before my mouth, etc. Watching and prayer are often joined together. We are best kept when recommended into God's hand. I do observe here, First, That unadvised and passionate speeches do easily drop from us in our troubles, especially in our persecution. Secondly, That a godly, conscientious man is very tender of these, as of all evil. He that would live in communion with God for the present, and hope to appear with comfort before him hereafter, is sensible of the least thing that tends to God's displeasure, and God's dishonour: this is the true spirit of one that will be owned by Christ at the last day. Thirdly, There is no way to prevent being provoked to impatience and rashness of speech, or any evil, but by keeping a watch, and renewing our obligations to God. Fourthly, Whoever would keep a watch must call in the aid and assistance of God's grace; "Lord, set a watch before my mouth." —Thomas Manton.