It was narrated that 'Amr bin 'As said:
"Do not corrupt the Sunnah of our Prophet Muhammad (ﷺ). The waiting period of an Umm Walad is four months and ten (days)." (TR)
English reference (TR): Vol. 3, Book 10, Hadith 2083 (TR)
Arabic reference (TR): Book 10, Hadith 2161 (TR)