I heard it mentioned in a sermon that Christians should do all things without complaining. Is this actually a command from God Himself or an expression of legalism?

You heard correctly. Philippians 2:14 gives us a bold imperative: Do all things without grumbling or arguing. In some of our English Bible versions, the word grumbling is translated as “complaining.” Grumbling is rooted in an attitude of resentment toward God about life circumstances. Even though the resentment is Godward, we often express our displeasure with negative comments about other people, most often behind their backs. Ironically, this grumbling drips from the redeemed lips of God’s people, even in the hallways and parking lots of churches. Some Christians may underestimate the damage that complaining does, but it hinders the cause of Christ and hurts people in the process.