Hazrat Abu Darda (may Allah be pleased with him) used to say during his speeches to the people that if a person reaches the morning and has not performed Witr, then his Witr will not be valid. Some people went and informed Hazrat Aisha (may Allah be pleased with her) about this, so she said that the Prophet (peace and blessings be upon him) would sometimes perform Witr even after the morning had come.