Muwatta Malik 1218

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، وَابْنِ، شِهَابٍ وَسُلَيْمَانَ بْنِ يَسَارٍ أَنَّهُمْ كَانُوا يَقُولُونَ عِدَّةُ الْمُخْتَلِعَةِ ثَلاَثَةُ قُرُوءٍ ‏.‏

Yahya related to me from Malik that he had heard that Said ibn al-Musayyab and Ibn Shihab and Sulayman ibn Yasar all said, "The idda of the woman with a khul divorce is three periods

Reference

Muwatta Malik 1218

In-book

Book 29, Hadith 15