Is a small amount of urine, such as a drop of urine on clothes, excused?

If a person knows there is urine on his garment, even a tiny drop, prayer with that garment is forbidden for him, and he must remove the impurity before praying, due to the Almighty's saying: (And your garments purify). As for the ruling on the impurity remaining on the garment without prayer, there is no sin in that. However, it is better for a Muslim to maintain the purity of his inner and outer self, following the example of the Prophet (peace be upon him) and his companions.