Fajr prayer called while praying witr.
Question

                If a person is praying witr (final odd prayer of late night) and during his 
prayer the muathin (caller of prayer) called for fajr (dawn) prayer, 
should he continue with his witr?
Praise be to Allah.Yes, if the call to prayer was during his witr, he should 
  complete the prayer and he is excused. This matter is part of the case of the 
  timing of witr prayer: i.e., does witr end at the start of the 
  timing of the fajr prayer or at the end of the timing of the fajr 
  prayer? The majority of scholars (al-jumhoor) are in agreement that answer 
  is the start of fajr prayer.1
From the book What Should You Do in 
  the Following Situations... ?
1 Is'aaf Ahl il-Asr bi-ma Warada 
  fi Ahkaam Salaat il-Witr, by Faihan al-Mutairi, p.33.