Sahih Muslim Book 6, Hadith Number 2387.

Chapter : The month may consist of twenty-nine days.

Umm Salama (Allah be pleased with him) reported that the Apostle of Allah (may peace be upon him) took an oath that he would not go to some of his wives for the whole of the month. When twenty-nine days bad passed he (the Holy Prophet) went to them in the morning or in the evening. Upon this it was said to him: Apostle of Allah, you took an oath that you would not come to us for a month, whereupon he said: The month may also consist of twenty-nine days.