Musnad Ahmed: Hadith 3630

The Messenger of Allah (ﷺ) said: "None of you should let the adhan of Bilal keep him from his sahoor, because he gives the adhan so that those of you who are praying qiyam may go back (to rest), and those of you who are asleep should wake up It is not when it is like this"- and he put his fingers together and raised (his hand - to indicate vertical) - “rather it is until it is like this"- and Yahya spread his forefingers apart (to indicate horizontal).