Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said concerning the words of Allah Almighty, "divorce them at (the start of) their waiting period" [Surah al-Talaq: 1]: that is, a state of purity in which there has been no intercourse. Some narrators added: or when the pregnancy has become evident.
I did not find this in the preserved (mahfuz) narrations.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14915