Ibn Majah

1؎: The Prophet (sallallahu alayhi wa sallam) indicated three times with the fingers of both hands, and on the third time, he folded one finger, so it amounted to twenty-nine days. The meaning of this is that the noble Companions (radi Allahu anhum) had said that twenty-two days have passed and eight days remain, from which it seemed that the month is always thirty days. This is not the case; it can also be twenty-nine days. And the Noble Prophet (sallallahu alayhi wa sallam) had taken an oath of abstention (ila) from the pure wives (radi Allahu anhun) for a month, then after twenty-nine days he returned to them. From this, it is understood that if someone swears an oath to do or not do something for a month, then acting upon the oath for twenty-nine days is sufficient.