Sahih Bukhari, Chapter: Witr Prayer

Once a person asked Allah's Apostle about the night prayer. Allah's Apostle replied, "The night prayer is offered as two Rakat followed by two Rakat and so on and if anyone is afraid of the approaching dawn (Fajr prayer) he should pray one Raka and this will be a Witr for all the Rakat which he has prayed before." Nafi' told that 'Abdullah bin 'Umar used to say Taslim between (the first) two Rakat and (the third) odd one in the Witr prayer, when he wanted to attend to a certain matter (during that interval between the Rakat).