The seven heavens and the seven earths in the palm of the Most Merciful are no more than a mustard seed in the hand of one of you
Al-Albaani reported from Ibn Taymiyyah that it is Sahih/Authentic, but he left no comment on itعن عبد الله بن عباس رضي الله عنهما قال: "ما السماوات السبع والأرضون السبع في كَفِّ الرحمن إلا كَخَرْدَلَةٍ في يد أحدكم".
‘Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported: "The seven heavens and the seven earths in the palm of the Most Merciful are no more than a mustard seed in the hand of one of you."
Explanation
Ibn ‘Abbās (may Allah be pleased with him) informs us in this reported citation that the seven heavens and the seven earths with all their greatness are, in proportion to Allah's palm, like the little mustard seed in the palm of one of us. This is a comparison of proportion, not a comparison of Allah's palm to the human palm, as the attributes of Allah do not resemble any of the attributes of His creation and He resembles none in His essence.
Source · Ibn Jareer - Adh-Dhahabi in Al-‘Uluww - ‘Abdullah ibn Ahmad ibn Hanbal
From the Hadith Encyclopedia (hadeethenc.com).