It was narrated from Ibn 'Umar that:
he divorced his wife when she was menstruating, and 'Umar mentioned that to the Prophet (ﷺ). He said: "Tell him to take her back then divorce her when she is pure (not menstruating) or pregnant." (PT)
English reference (PT): Vol. 3, Book 10, Hadith 2023 (PT)
Arabic reference (PT): Book 10, Hadith 2101 (PT)