عن أبي ذر جندب بن جنادة رضي الله عنه قال: اجتمعت غنيمة عند رسول الله صلى الله عليه وسلم فقال: «يا أبا ذر اُبْدُ فيها» فبَدَوْتُ إلى الرَّبذَة فكانت تُصيبني الجنابة فأمكث الخَمْسَ والسِّتَّ، فأتيتُ النبي صلى الله عليه وسلم فقال:«أبو ذر» فَسَكَتُّ فقال: «ثَكِلَتْكَ أمك أبا ذر لأمِّكَ الوَيْلُ» فدعا لي بجارية سوداء فجاءت بِعُسٍّ فيه ماء فستَرتْنِي بثَوب واستَتَرْتُ بالرَّاحلة، واغتسلتُ فكأني أَلقَيْتُ عني جَبَلًا فقال «الصعيدُ الطيِّبُ وُضُوءُ المسلم ولو إلى عشر سنين، فإذا وجدتَ الماء فأَمِسَّهُ جِلدَكَ فإن ذلك خَيرٌ». [صحيح] - [رواه أبو داود والترمذي والنسائي وأحمد] المزيــد ...
Abu Dharr Jundub ibn Junādah (may Allah be pleased with him) reported: A few goats were in the possession of the Messenger of Allah (may Allah’s peace and blessings be upon him) so he said: "O Abu Dharr, take them to the desert." I took them to the desert of Rabadhah, and I would sometimes be in a state of Janābah and stay for five or six days (without bathing). I came to the Prophet, may Allah’s peace and blessings be upon, and he said:"Abu Dharr?" I kept silent. He said:"May your mother lose you, Abu Dharr! Woe to your mother!" He then called a black maid and she brought me a big pot of water. She concealed me with a garment and I hid behind a camel and washed. It was as if I removed a mountain off myself. He then said:"Clean earth is ablution for the Muslim, even up to ten years. Then when you find water, wash your skin with it, for that is better." Sahih/Authentic. - [At-Tirmidhi]
This Hadīth sheds light on one of the aspects of facilitation that Islamic Shariah brings to people, which is making Tayammum when water is not available. Clean dust serves in place of water, which is used in regular ablution.
The exact number of years mentioned in the Hadīth is not intended per se; rather, it means that Tayammum is valid so long as water is unavailable, regardless of the time interval whether it is ten or twenty or thirty or more. This is also true when water is available but it cannot be used for religious or medical reasons.
Although purification by Tayammum is weaker, it substitutes ablution with water as a necessity to perform prayer before the time of prayer ends.
The concession to make Tayammum expires the moment water becomes available and one is able to use it. Therefore, the Prophet (may Allah's peace and blessings be upon him) guided Abu Dharr to go back to the original option – which is using water – when it is available. One has then to let water touch his skin; i.e. make water run over his skin in ablution or take a ritual bath. Tayammum expires when one sees water and is able to use it. The ability to use water is what is intended by the 'availability' of water.