Hadith: Allah hears he who praises Him. O Allah, our Lord, to You is the praise as much as fills the heavens, as much as fills the earth, and as much as fills whatever You will after that

عن ابن أبي أوفى قال: كان رسول الله صلى الله عليه وسلم إذا رفع ظهره من الركوع قال: «سمع الله لمن حمده، اللهم ربنا لك الحمد، ملء السماوات وملء الأرض وملء ما شئت من شيء بعد».
[صحيح] - [رواه مسلم]

Ibn Abi Awfa (may Allah be pleased with him) reported: When the Messenger of Allah (may Allah’s peace and blessings be upon him) raised his back from rukū‘, he would say: "Sami‘a allahu liman hamidah, allahumma rabbana laka al-hamd mil’ as-samawati wa mil’ al-ardi wa mil’ ma shi’ta min shay’in ba‘d" (Allah hears he who praises Him. O Allah, our Lord, to You is the praise as much as fills the heavens, as much as fills the earth, and as much as fills whatever You will thereafter).
Sahih/Authentic. - [Muslim]

Explanation

Whenever the Prophet (may Allah’s peace and blessings be upon him) raised his back from rukū‘ in prayer, he would say: Allah hears he who praises Him. 'Hear' here means: respond. So, it means that when a person praises Allah Almighty, He responds to him and accepts the praise from him and rewards him for it. The praise is as much as fills the heavens, as much as fills the earth, and as much as fills whatever You will after that - i.e., the Throne and the Kursī and other things, which are within Allah's power. It is recommended to say this dhikr upon rising from rukū‘. It is legitimate for everyone - the Imām, those led in prayer, or those praying alone - to say "Allah hears he who praises Him" and the response to it.