How (will it be) then when We bring from each nation a witness and We bring you (O Muhammad) as a witness against these people? (V.4:41)

The necklace of Asma' was lost, so the Prophet (ﷺ) sent some men to look for it. The time for the prayer became due and they had not performed ablution and could not find water, so they offered the prayer without ablution. Then Allah revealed (the Verse of Tayammum).