What does the Prophet Muhammad (peace be upon him) tell us about distinguishing the white thread from the black thread, according to the revelation in Surah al-Baqarah?
 Narrated Sahl bin Saud:                     When the following verses were revealed: 'Eat and drink until the white thread appears to you, distinct from the black thread' and of dawn was not revealed, some people who intended to fast, tied black and white threads to their legs and went on eating till they differentiated between the two. Allah then revealed the words, 'of dawn', and it became clear that meant night and day.