Al-Kāfi - Volume 3 > One’s having Little Water on a Journey but Is Afraid of Facing Thirst
Hadith #4

4ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنْ عَبْدِ الله بْنِ الْمُغِيرَةِ قَالَ إِنْ كَانَتِ الأرْضُ مُبْتَلَّةً وَلَيْسَ فِيهَا تُرَابٌ وَلاَ مَاءٌ فَانْظُرْ أَجَفَّ مَوْضِعٍ تَجِدُهُ فَتَيَمَّمْ مِنْ غُبَارِهِ أَوْ شَيْ‏ءٍ مُغْبَرٍّ وَإِنْ كَانَ فِي حَالٍ لاَ تَجِدُ إِلاَّ الطِّينَ فَلاَ بَأْسَ أَنْ تَتَيَمَّمَ بِهِ.

4. Ali ibn Ibrahim has narrated from his father from ‘Abd Allah ibn al-Mughirah who has said the following: “(Abu ‘Abd Allah, ‘Alayhi al-Salam ) has said, ‘If the ground is wet and dry soil or water is not available you must look for an area which is the most dry of the others, take Tayammum with dust thereof or something with dust on it. If you can find nothing else other than clay, then it is not harmful to take Tayammum therewith.’”