Al-Matalib al-'Aliyah 157 — Tayammum

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that "the Prophet (peace and blessings be upon him) was on a journey, and when the time of prayer came, all the people dismounted from their mounts. A shepherd saw them, so he too came down (from a hill or height), struck his hand on the clean earth and performed tayammum, and then gave the call to prayer (adhan)."
There is weakness in the chain of this hadith.