Musnad Ahmad 3694

It was narrated from Ibn Mas'ood (رضي الله عنه) that The Messenger of Allah (ﷺ) said: “None of you should let the adhan of Bilal keep him from his sahour, 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 may wake up. It is not when it is like this; rather it is until it is like this" - and Ibn Abi ‘Adiyy Abu ‘Amr put his fingers together and held them pointing down (to indicate vertical) -and he spread his forefingers apart (to indicate horizontal), i.e. the dawn.