When is the recommendation for using the Siwak particularly emphasized?

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The recommendation to use the Siwak is strongly emphasized in three specific instances. First, before prayer, based on earlier reports. Second, immediately upon waking up, whether from night or day sleep, because the mouth's odor changes due to the mouth being closed during sleep. This is supported by Hadith stating the Prophet would use the Siwak before performing Wudu (ablution) after waking (reported by Abu Dawud). Third, when the mouth's odor changes due to eating or other causes, as its purpose is to remove bad odors and provide fragrance.