Musnad Ahmad 937:218

Musa ibn Dawud narrated to us, Ibn Lahi'ah narrated to us, from Salim ibn Ghaylan, from Sulayman ibn Abi Uthman, from Adi ibn Hatim al-Homsi, from Abu Dharr, that the Prophet, peace and blessings of Allah be upon him, said to Bilal: "You, O Bilal, call the adhan when the dawn is shining in the sky, but that is not the dawn. The dawn is only like this, horizontally." Then he called for his suhoor and ate suhoor. And he used to say: "My nation will continue to be in goodness as long as they delay the suhoor and hasten the iftar."