Tayammum is a concession that Allah (may He be exalted) has prescribed for His slaves when water is not available or it is not possible to use it, so as to make things easy for them, as it makes it permissible to offer the prayer and do other acts of worship. So if a Muslim is not in a state of purity, and cannot find water, but he wants to mention the name of Allah (may He be exalted), then he may do tayammum before mentioning Him, as is indicated by this hadith. Abu Juhaym ibn al-Harith ibn al-Simmah al-Ansari narrates that when the Prophet (blessings and peace of Allah be upon him) came from the direction of Bi’r Jamal – which is a place near Madinah – he was met by a man who greeted him with salaam, but he did not return the greeting until he turned to a wall, patted it with his hands, and wiped his face and hands, then he returned the man’s greeting. That was because al-Salaam is one of the names of Allah (may He be exalted), so the Prophet (blessings and peace of Allah be upon him) wanted to mention the name of Allah when he was in a state of purity. Hence he did tayammum, then he returned the man’s greeting. This hadith indicates that tayammum may be done for supererogatory actions and virtuous deeds, and not only for obligatory deeds. It indicates that tayammum may be done when not travelling, if there is no water available, or if one is not able to use it. It indicates that tayammum consists of patting the dust once to wipe the face and hands..