Muwatta Malik 1168

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ كَانَ يَقُولُ أَيُّمَا رَجُلٍ آلَى مِنِ امْرَأَتِهِ فَإِنَّهُ إِذَا مَضَتِ الأَرْبَعَةُ الأَشْهُرِ وُقِفَ حَتَّى يُطَلِّقَ أَوْ يَفِيءَ وَلاَ يَقَعُ عَلَيْهِ طَلاَقٌ إِذَا مَضَتِ الأَرْبَعَةُ الأَشْهُرِ حَتَّى يُوقَفَ ‏.‏

Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, "When a man makes a vow to abstain from intercourse with his wife and four months have passed he must declare his intent and either he is divorced or he revokes his vow. Divorce does not occur until four months have passed and he continues to abstain