Is it permissible to use toothpaste as a substitute for the siwak when performing ablution, and will the one who does so be rewarded for it?

The "siwak" is one of the Prophetic Sunnahs (traditions) that encompasses any tool used to clean the teeth and achieve the intended purpose with the intention of following the Sunnah (definition). This includes the toothbrush. Linguistically, "siwak" means rubbing the teeth. The Prophet, peace be upon him, used it from the Arak tree and from date palm branches, and he did not specify a particular tree for the siwak. The jurists did not restrict the siwak to the Arak stick; rather, it includes anything that cleans the mouth. The siwak is an understandable act of worship, and its purpose is to clean the teeth and freshen the mouth, which can be achieved with any tool. Using a toothbrush and toothpaste suffices for the siwak and earns reward, but using an Arak stick is better for following the Sunnah and its ease of portability.