بُلُوغُ الْمَرَامِ
Bulugh al-Maram 174
The Book of Prayer
وَعَنْ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا أَنَّ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -قَالَ: { لَا صَلَاةَ بَعْدَ اَلْفَجْرِ إِلَّا سَجْدَتَيْنِ } أَخْرَجَهُ اَلْخَمْسَةُ, إِلَّا النَّسَائِيُّ [1] .
Narrated Ibn 'Umar (RA): Allah's Messenger (ﷺ) said: "There is no Salat (i.e. voluntary prayer) after the break of dawn except Sajdatain (Sunnah of Fajr)." [Reported by al-Khamsah except an-Nasa'i].
Sunnah.com reference: Book 2, Hadith 27