بُلُوغُ الْمَرَامِ
Bulugh al-Maram 402
The Book of Prayer
وَعَنْهُ قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ أَثْقَلُ اَلصَّلَاةِ عَلَى اَلْمُنَافِقِينَ: صَلَاةُ اَلْعِشَاءِ, وَصَلَاةُ اَلْفَجْرِ, وَلَوْ يَعْلَمُونَ مَا فِيهِمَا لَأَتَوْهُمَا وَلَوْ حَبْوًا } مُتَّفَقٌ عَلَيْه ِ [1] .
Narrated [Abu Hurairah (RA)]: Allah's Messenger (ﷺ) said: "The most burdensome prayers for hypocrites are the 'Isha' and the Fajr prayers and if they knew what (rewards) these (prayers) contain, they would have come to them (in the mosques), even though they had to crawl on their knees." [Agreed upon].
Sunnah.com reference: Book 2, Hadith 306