Dua after Fajr salah
Juwairiya reported that Allah's Messenger (ﷺ) came out from (her apartment) in the morning as she was busy in observing her dawn prayer in her place of worship. He came back in the forenoon and she was still sitting there. He (the Holy Prophet) said to her:
You have been in the same seat since I left you. She said: Yes. Thereupon Allah's Apostle (ﷺ) said: I recited four words three times after I left you and if these are to be weighed against what you have recited since morning these (words) would outweigh them and (these words) are:" Hallowed be Allah and praise is due to Him according to the number of His creation and according to the pleasure of His Self and according to the weight of His Throne and according to the ink (used to write) words (for His Praise)."
سُبْحَانَ اللَّهِ وَبِحَمْدِهِ عَدَدَ خَلْقِهِ وَرِضَا نَفْسِهِ وَزِنَةَ عَرْشِهِ وَمِدَادَ كَلِمَاتِهِ
Reference: Sahih Muslim 2726