Bulugh al-Maram 2 The Book of Prayer كتاب الصلاة 302

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 . 1 – صحيح. رواه البخاري (657)، ومسلم (651).

Narrated [Abu Hurairah (RA)]: Allah’s Messenger (صلى الله عليه وسلم) 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].