Sunan an-Nasa'i, Hadith 3525 ⓘ
It was narrated from 'Aishah that the Messenger of Allah said: "It is not permissible for a woman to mourn for anyone who dies for more than three days, except for her husband."
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " لاَ يَحِلُّ لاِمْرَأَةٍ تَحِدُّ عَلَى مَيِّتٍ أَكْثَرَ مِنْ ثَلاَثٍ إِلاَّ عَلَى زَوْجِهَا " .
also referenced under: Book 27, Hadith 137 (In-Book) & Volume 4, Book 27, Hadith 3555 (DarusSalam)