Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 493
Narrated Abu Jamra:
I asked Aidh bin Amr, who was one of the companions of the Prophet one of those (who gave the allegiance to the Prophet the Tree: "Can the Witr prayer be repeated (in one night)?" He said, "If you have offered it in the first part of the night, you should not repeat it in the last part 'of the night." (See Fateh-al-Bari page 458 Vol 8th).