This is not a hadith. It is a statement attributed to Ishaq ibn Rahwayh and the Hanbalis.
The meaning of the statement is that the minimum amount of the Quran that should be recited every day is one fortieth of the whole. In other words one should complete the whole Quran every forty days. This is based on the hadith:
عن عبد الله بن عمرو، أنه سأل النبي صلى الله عليه وسلم، كم يقرأ القرآن؟ قال: في أربعين يوما، ثم قال: في شهر
Abdullah ibn Amr asked the Prophet (ﷺ); In how many days should one complete the recitation of the Qur'an? He said: In forty days. He then said: In one month ...
— Sunan Abi Dawud
This has no relation to the 30 Juz of the Quran. Keep in mind that this division into 30 parts is not God given, rather it was made much later by the people to make it easy to complete the Quran within Ramadan. It is an arbitrary division and anyone can divide the Quran into 10 or 30 or 40 or 80 or 100 parts or any other number, it makes no difference.