Muwatta Malik 1219

Arabic

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

Translation

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)

Authenticity

Sahih · Salim al-Hilali

Reference