1- Confirmed sunna before wudu and prayer.

Abu Hurayrah (G. p. H) narrated that the Prophet (P. B. U. H) said:

"Were I not afraid that it would cause hardship to my Umma [Nation]. I would have ordered them to use siwak before each prayer"

[Muslim, Vol. 1, The Book of Purification, Chapter 15/42].

Also, on rising from sleep in the day or night.

Hut Hayfa (G. p. H) said: "The Prophet (P. B. U. H) would brush his teeth with siwak whenever he woke up at night".

[Muslim, Vol. 1, The Book of Purification, Chapter 15/46].

Siwak is also recommended for bad breath due to not talking for a long time, not eating for a long period, or eating something which has a bad odor like garlic and onions.

2- Recommended for reading Quran, making thikr, or when one's teeth become discolored.

Thammam ibn Al-Abbas (G. p. H) narrated that the Prophet (P. B. U. H) said:

"Why do you come to me with plaque on your teeth, use siwak".

[Musnad Al-Imam Ahmad, Vol. 1, page 412].

Siwak is also recommended before going to sleep and when entering one's home.

Aisha (G. p. H) narrated that: "When the Prophet (P. B. U. H) came home, he would start with brushing his teeth with siwak".

[Muslim, Vol. 1, The Book of Purification, Chapter 15/44]

Finally, siwak is recommended when entering a mosque.

3- Makrooh for a fasting person after mid-morning because Abu Hurayrah (G. p. H) narrated that the Prophet (P. B. U. H) said:

"By him in whose hand is my soul ! The breath of a fasting person is sweeter to Allah (G. B. H) than the smell of musk".

[Al-Bukhari, Vol. 2, The Books of Fasting, Chapter 9/1805].

Using the word sweeter is proof that it should be kept.

Also, since it is the result of an act of worship, it is makrooh to cleanse it just like the blood of martyrs.