عن ابن عمر قال بينما نحن نصلي مع رسول الله إذ قال رجل من القوم : الله أكبر كبيرا والحمد لله كثيرا وسبحان الله بكرة وأصيلا ، فقال رسول الله صلى الله عليه وسلم : من القائل كلمة كذا وكذا ؟ فقال رجل من القوم : أنا يا رسول الله قال : عجبت لها ، فُتحت لها أبواب السماء
قال ابن عمر : فما تركتهنّ منذ سمعت رسول الله يقول ذلك
From Ibn ‘Umar Raḍi-Allāhu 'Anhumā who said: Whilst we were praying with the Messenger of Allāh ﷺ a man from amongst people recited;
Allaahu Akbaru Kabeeran Wal Ḥamdulillaahi Katheeran Wa SubḥaanAllaahi Bukratan Wa Aṣeelaa,
(Allāh is Great, the Greatest, and praise belongs to Allāh in abundance, and glorified be Allāh at the beginning and end of the day),
So the Messenger of Allāh ﷺ asked (after prayer): “Who said such and such?!!”
The man replied: “I did O Messenger of Allāh.”
So the Messenger of Allāh ﷺ said: “I was amazed by it, for it opened the gates of heaven.”
Ibn 'Umar Raḍi-Allāhu 'Anhumā said: “I have not abandoned them (i.e these words) since I heard the Messenger of Allāh ﷺ saying that.”
● [صحيح مسلم]
Note: Sh. al-Albānī Raḥimahullāh mentioned in his book (صفة صلاة النبي) that this Dhikr is from one of the supplications with which the prayer is opened (after the first Takbeer).