Is it permissible to pray in a place that is level, yet has a depressed and yellowed section due to frequent standing on it?

The discoloration of the foot placement area or its sinking below the level of the mattress due to prolonged standing does not affect the validity of the prayer performed therein. The matter is broad, as the Prophet (peace be upon him) prayed on a mat that had turned black from extensive use. However, if the place is uneven in a way that prevents the limbs from assuming the correct posture, it is better to pray in an even place. It is disliked to stand on one leg, or to join the feet together, or to place one foot in front of the other without an excuse, as this contradicts khushu' (devotion and humility).