Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from the Noble Prophet (peace and blessings of Allah be upon him) that he (peace be upon him) said: "Sayyiduna Bilal (may Allah be pleased with him) gives the call to prayer at night, so you should continue eating and drinking (for suhoor) until Sayyiduna Ibn Umm Maktum (may Allah be pleased with him) gives the call to prayer (then you should stop)." Ubaidullah says that I heard Qasim narrate this hadith from Sayyidah Aisha (may Allah be pleased with her), he says that the interval between the two calls to prayer was such that one would descend (after giving the adhan) and the other would ascend (to give the adhan).