The Messenger of Allah said, 'Using the siwak' purifies the mouth and is pleasing to the Lord,' and he made it one of the confirmed practices. It has benefits for both the outward and the inward being which even men of intelligence cannot count.
As you remove the stains caused by food and drink from your teeth with the siwak, so remove the impurity of your wrong actions by humble entreaty, humility, night prayers, and asking for forgiveness before dawn. Purify your outer being from impurities, and your inner being from the turbidity of acts of opposition and committing anything prohibited, all the while acting sincerely for Allah. The Holy Prophet made its use an example for people for alertness and attention, in that the siwak is a clean, soft plant and the twig of a blessed tree. The teeth are what Allah created in the mouth as a tool for eating, an implement for chewing, a reason for enjoying food and for keeping the intestines in order. The teeth are pure jewels, which become dirty because they are present when food is chewed, leading to a deterioration in the way the mouth smells, and decay in the gums. When the intelligent believer cleans with the soft plant and wipes it on these pure jewels, he removes the decay and adulteration from them and they then revert to their original state.
Similarly Allah created the heart pure and clean, and made its food remembrance, reflection, awe and respect. When the pure heart turns grey by being fed on heedlessness and vexation, it is polished by the burnish of repentance and cleaned by the water of regret, so that it reverts to its primal state and its basic essence. As Allah said,
Surely Allah loves those who turn [in repentance] to Him, and He loves those who purify themselves. (2:222)
In recommending the use of the siwak the Holy Prophet was advocating that it be used on the teeth themselves; but he also implied the meaning and example we have referred to above, that for anyone who empties his faculty of reflection for the purpose of drawing inward lessons from the outward examples, with respect to both the principle and roots of faith, Allah will open the springs of wisdom, and will give him still more of His overflowing favour, for Allah does not neglect the reward of those who act well.