In Alma 26, Ammon celebrates with his brothers regarding their missionary successes among the Lamanite people and how God has used them to bring many souls to salvation. He is obviously excited that they have been personally used for this accomplishment, stating, "If we had not come up out of the land of Zarahemla, these our dearly beloved brethren…would still have been…strangers to God" (verse 9).
At this point, his brother Aaron rebukes him, saying, "Ammon, I fear that thy joy doth carry thee away unto boasting" (verse 10). Ammon replies, "I do not boast in my own strength…but I will boast of my God, for in his strength I can do all things" (verses 11-12). He then continues his rejoicing and "boasting in the Lord."
What's the difference between boasting in the Lord and just plain boasting? It all comes down to motivation.