Is it obligatory to wear one's best clothes when going to the mosque, and is going to the mosque for prayer in a slightly dirty garment considered a sin?

It is more complete for the worshiper to come to prayer in the best appearance and most beautiful clothes, as Allah Almighty says: "O Children of Adam, take your adornment at every place of prayer." For Allah is more deserving of adornment. However, if the worshiper goes to the mosque with a dirty but pure garment, his prayer is valid and there is no sin upon him, even if that is contrary to what is preferable. There is no harm in praying in a garment that has sweat or non-impure dirt, or an old garment that covers the 'awrah.