Musnad Ahmad 26:170

Ibn Abi ‘Adi narrated to us, from Sulayman, from Abu ‘Uthman, from Ibn Mas‘ud, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not the Adhan of Bilal prevent any one of you from his Suhur, for he only calls out—or he said: gives the Adhan—so that your standing one may return, and your sleeping one may be awakened. It is not that he says it is like this, but rather until he says it is like this." And Ibn Abi ‘Adi—Abu ‘Amr—clenched his fingers and pointed them downwards, and spread apart his two index fingers, meaning the dawn.