Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) saw a person sitting aloof and not praying with the people. He asked him: “O so-and-so, what prevented you from offering the prayer with the people?” He replied: “O Messenger of Allah, I am ritually impure and there is no water.” In response, the Prophet (may Allah’s peace and blessings be upon him) said: “You should use clean earth, and that will be sufficient for you.”

عن عمران بن حصين رضي الله عنهما أن رسول الله صلى الله عليه وسلم رأى رجلًا مُعتزلًا، لم يُصَلِّ في القوم، فقال: (يا فلان، ما منعك أن تصلي في القوم؟) فقال: يا رسول الله أصابتني جنابةٌ، ولا مَاءَ، فقال: (عليك بالصَّعِيدِ، فإنه يَكْفِيَكَ).
[صحيح] - [متفق عليه]

‘Imrān ibn Husayn (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) saw a person sitting aloof and not praying with the people. He asked him: "O so and so, what prevented you from offering the prayer with the people?" He replied: "O Messenger of Allah, I am ritually impure and there is no water." In response, the Prophet (may Allah’s peace and blessings be upon him) said: "You should use clean earth, and that will be sufficient for you."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) led his Companions in the Fajr prayer. When he finished the prayer, he noticed a man that had not prayed with them. Out of his remarkable gentleness and good call to Allah, the Almighty, he did not rebuke the man for having failed to attend the congregational prayer until he could know the reason behind this. So, he asked the man: “O so-and-so, what prevented you from praying with the congregation?” The man clarified his excuse, according to his reasoning. He said that he was in a state of ritual impurity and had no water; that is why he delayed the prayer until he could find water and purify himself therewith. Thereupon, the Prophet (may Allah's peace and blessings be upon him) informed him that Allah, the Almighty, out of His kindness, provided a replacement for water when it comes to purification; and that is clean earth. He told him to use it instead of water, which was unavailable.