Sahih Bukhari Volume 2, Book 22, Hadith Number 327.

Narated By Asma’ : I went to ‘Aisha and she was standing praying and the people, too, were standing (praying). So I said, “What is the matter with the people?” She beckoned with her head towards the sky.

I said, “(Is there) a sign?” She nodded intending to say, “Yes.”