Sahih Bukhari Volume 007, Book 072, Hadith Number 770.

Narrated By 'Aisha : A necklace belonging to Asma' was lost, and the Prophet sent men in its search. The time for the prayer became due and they were without ablution and they could not find water; therefore they prayed without ablution, They mentioned that to the Prophet. Then Allah revealed the Verse of Tayammum. ('Aisha added: that she had borrowed (the necklace) from Asma').