The verse: {...You may eat and drink until the white thread appears to you distinct from the black thread..} [Al-Baqarah: 187] was revealed, but (at dawn) was not. So when people wanted to fast, one would tie a white thread and a black thread around his feet, and continues eating until he can clearly see them to him. After that, Allah revealed (at dawn), so they knew that it meant night and day.

Bilal calls to prayer -first call- while it is still night, so you may eat and drink until Ibn Um Maktoom calls to prayer -second call.

We were with the Messenger of Allah (PBUH) on a journey while he was fasting. When the sun had set, he said: O Bilal, come down and prepare some soup for us. Bilal said: O Messenger of Allah, will you wait until it is evening. The Prophet (PBUH) said: Come down and prepare some soup for us. Bilal said: O Messenger of Allah, it is still daytime. The Prophet (PBUH) said: Come down and prepare some soup for us. So he came down and prepared the soup. So the Messenger of Allah (PBUH) drank and then said: When you see the night falling from there, the a fasting person should break his fast. And he pointed with his finger towards the east.