Musnad Abu Ya'la al-Mawsili 154:30

Shayban ibn Farrukh narrated to us, Sa'id ibn Rashid narrated to us, from 'Ata', from Ibn 'Umar: That the Prophet, may Allah bless him and grant him peace, was on a journey of his. When the prayer time arrived, the people dismounted. A shepherd saw them, so he dismounted and struck the earth with his hand, then he performed Tayammum. Then he gave the Adhan. He said: "Allah is the Greatest, Allah is the Greatest." The Prophet of Allah, may Allah bless him and grant him peace, said: "Upon the Fitrah." He said: "I bear witness that there is no god but Allah." He said: "He has exited the Fire."