Sunnah.com reference : Book 8, Hadith 147English translation : Book 8, Hadith 1102Arabic reference : Book 8, Hadith 1091
Narrated Ibn 'Abbas (RA):
The swearing to stay away from one's wife in al-Jahiliyyah era was for one or two years. Then, Allah appointed a period of four months for it. So, if it is less than four months, it is not swearing to stay away from one's wife.

[al-Baihaqi reported it]
وَعَنِ اِبْنِ عَبَّاسٍ رَضِيَ اَللَّهُ عَنْهُمَا قَالَ: { كَانَ إِيلَاءُ اَلْجَاهِلِيَّةِ اَلسَّنَةَ وَالسَّنَتَيْنِ, فَوَقَّتَ اَللَّهُ أَرْبَعَةَ أَشْهُرٍ, فَإِنْ كَانَ أَقَلَّ مِنْ أَرْبَعَةِ أَشْهُرٍ, فَلَيْسَ بِإِيلَاءٍ }  أَخْرَجَهُ اَلْبَيْهَقِيُّ .‏  1‏ .‏‏1 ‏- صحيح.‏ رواه البيهقي (7 /381)‏.‏