Performing Tayammum instead of Istinja'

A: Performing Istinja' is not necessary for making Wudu'; rather, it is obligatory after passing urine or stools. Moreover, it is not permissible to perform Tayammum in the presence of water, even if the weather is very cold unless one fears getting severely ill by using water. Also, it is not permissible to resort to Tayammum due to the lack of a covered place, as your colleague can cover you with a piece of cloth or the like when necessity demands it.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.