This has been stated in the verse: وَلَوْ أَنَّمَا فِي الْأَرْ‌ضِ مِن شَجَرَ‌ةٍ أَقْلَامٌ (27) through a similitude. Says the similitude: 'If all trees that are on the earth were to be pens (or pens were to be made of all branches on them) and the ocean (converted into ink) is supported by seven seas following it, the words of Allah would (still) not come to an end.' The expression: کلمات اللہ (kalimatul-lah, i.e. the words of Allah) means His knowledge and wisdom (Ruh ul-Maani and Mazhari) which includes the manifestations of Divine power and Divine blessings. Then, the 'seven seas', referred to here, do not mean that there are seven seas present elsewhere. Instead of that, this is a manner of saying: Suppose if this ocean were to be replenished with seven more oceans, still then, all these words of Allah could not be committed to writing. Even the number of 'seven' appears here as a part of the example. No restriction is intended. Another verse of the Qur'an proves it. There it was said:

قُل لَّوْ كَانَ الْبَحْرُ‌ مِدَادًا لِّكَلِمَاتِ رَ‌بِّي لَنَفِدَ الْبَحْرُ‌ قَبْلَ أَن تَنفَدَ كَلِمَاتُ رَ‌بِّي وَلَوْ جِئْنَا بِمِثْلِهِ مَدَدًا

Say, 'If the ocean were to be ink for the Words of my Lord, the ocean would have been consumed before the Words of my Lord are exhausted, even though we were to bring another one, like it, in addition' - Al-Kahf, 18:109.

In this verse, by saying: بِمِثْلِهِ (bimithlihi: like it), a clear hint has been given that, no matter how many oceans are supposed, their collective writing fluid cannot circumscribe the Words of Allah. Rationally, the reason is obvious. The oceans may be added one after the other and be they seven or seven thousand, they shall still remain limited while the Words of Allah (knowledge) are unlimited. How, then, can something limited make an all-encompassing coverage of what is unlimited?

As it appears in some narratives, this verse was revealed in response to a question posed by Jewish priests. The reason which prompted them to ask this question was the verse of the Qur'an where it has been said: وَمَا أُوتِيتُم مِّنَ الْعِلْمِ إِلَّا قَلِيلًا (and you are not given but a little from the knowledge - A1-'Isra', 17:85). When the Holy Prophet ﷺ came to the blessed city of Madinah, some Jewish priests visited him and it was about this verse that they confronted him by saying, "You say that you have been given a little from the knowledge. Is this what you are saying about your people, or have you included us too therein?" The Holy Prophet ﷺ said, "I mean all," that is, 'our people and the Jews and Christians as well.' Thereupon, increasing the tempo of their opposition, they said, "To us, Allah Ta’ ala has given the Torah which stands out as: تِبْيَانًا لِّكُلِّ شَيْءٍ (an explanation of everything)." He said, "That too is but a little as compared to Divine Knowledge. Then, even the total knowledge contained in the Torah is not known to you either - what you go by is no more than a certain measure of what you need. Therefore, as compared to the Divine Knowledge, the collective knowledge of all Scriptures and Prophets is also nothing but a little." It was to support this statement that the verse: وَلَوْ أَنَّمَا فِي الْأَرْ‌ضِ مِن شَجَرَ‌ةٍ أَقْلَامٌ (And if all trees that are on the earth were to be pens... - 27) was revealed. (Ibn Kathir)