From Sahih Bukhari (Hadith 1235):
Narrated 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
Reference: Bukhari 1235