Muwatta Malik 1219

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ سَمِعَ ابْنَ شِهَابٍ، يَقُولُ عِدَّةُ الْمُطَلَّقَةِ الأَقْرَاءُ وَإِنْ تَبَاعَدَتْ ‏.‏

Yahya related to me from Malik that he heard Ibn Shihab say, "The idda of the divorced woman is reckoned by the menstrual cycles even if she is estranged ." (The reason the idda is normally reckoned by the menstrual cycle is to see whether the woman is pregnant or not)

Reference

Muwatta Malik 1219

In-book

Book 29, Hadith 16