Ibn Mas`ud (May Allah be pleased with him) reported: Messenger of Allah (Peace and blessings be aupon him) said, “I met Ibrahim (Peace and blessings be aupon him) on the Night of Ascension (Al-Asra), and he said to me: `O Muhammad, convey my greetings to your Ummah, and tell them that Jannah has a vast plain of pure soil and sweet water. It is a plain levelled land. The plants grow there by uttering: Subhan-Allah, Al-hamdu lillah, La ilaha illallah and Allahu Akbar (Allah is free from imperfection; praise be to Allah; there is no true god except Allah; and Allah is Greatest).”[At-Tirmidhi].
Commentary: Qi`an is the plural of Qa` which means plain levelled land that does not have any tree. Trees grow on the plain land of Jannah in return for remembrance and Glorification of Allah. The more one remembers Allah, the greater is the number of trees which grow on the piece of land that will be awarded to him.
Source: Riyad-us-Saliheen
Pearls of a Muslim 