Musnad Ahmad 1094

The Mother of the Believers ‘Ā'ishah (may Allah be pleased with her) mentions the time when the Prophet (may Allah's peace and blessings be upon him) used to pray Witr at night. He used to pray it at any time at night, without restricting himself to a certain time. Sometimes he would pray it at the beginning of the night after praying ‘Ishā'.

Sometimes he would pray in the middle of the night after the first third of the night had passed. Sometimes he would pray in the last part of the night after two thirds had passed, even in the last hour of the night.