Musnad Ahmad 648:10

Ya'qub narrated to us, my father narrated to us, from Salih, who said: Ibn Shihab said: 'Ubayd Allah ibn 'Abd Allah narrated to me, from Ibn 'Abbas, from 'Ammar ibn Yasir, that the Messenger of Allah (may Allah bless him and grant him peace) camped with the vanguard of the army, and with him was 'Aishah, his wife. A necklace of hers made of Zafari onyx broke, so he detained the people seeking that necklace of hers until dawn broke, and the people had no water. Then Allah the Almighty and Majestic revealed to His Messenger (may Allah bless him and grant him peace) the concession of purifying oneself with pure soil. The Muslims stood up with the Messenger of Allah (may Allah bless him and grant him peace), and they struck the earth with their hands, then they raised their hands, and they did not grasp anything of the dust. They wiped their faces and their hands up to the shoulders, and from the palms of their hands to the armpits. Let not people be deceived by this. And it reached us that Abu Bakr said to 'Aishah (may Allah be pleased with them both): "By Allah, I did not know that you were blessed."