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." (DE)
English reference (DE): Vol. 3, Book 10, Hadith 2023 (DE)
Arabic reference (DE): Book 10, Hadith 2101 (DE)