What is meant by "Khardalah" in Ibn Abbas's saying, "The seven heavens and the seven earths are in the hand of the Most Merciful like nothing but a mustard seed in the hand of one of you"? Is it the small atom used in science, or the insect? And is Paradise considered small in the hand of the Most Merciful like a mustard seed? And is Ibn Abbas's saying a Hadith from the Prophet, peace be upon him, and what is its authenticity?

A mustard seed is one mustard, which is a well-known plant used as an idiom for smallness, and its weight is four times that of an atom.

What is intended by what is mentioned regarding the heavens and the earth is to show the greatness of the Creator, as Paradise and other creations share in this.

Paradise is larger than the heavens and the earth. The proof for this is the saying of Ibn Abbas (may Allah be pleased with them both): "The seven heavens and the seven earths in the hand of Allah are nothing but like a mustard seed in the hand of one of you." This statement is attributed to Ibn Abbas (mawquf) and not مرفوع (directly attributed to the Prophet, peace be upon him). It was narrated by Al-Tabari and Ibn Ahmad.