Reference : Hisn al-Muslim 251

وَقَالَ صلى الله عليه وسلم: أَقْرَبُ مَا يَكُونُ الرَّبُّ مِنَ الْعَبْدِ فِي جَوْفِ اللَّيْلِ الآخِرِ فَإِنِ اسْتَطَعْتَ أَنْ تَكُونَ مِمَّنْ يَذْكُرُ اللَّهَ فِي تِلْكَ السَّاعَةِ فَكُنْ
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Allah's Messenger (ﷺ) said: "The closest that the Lord comes to the slave is in the last portion of the night. So, if you are able to be among those who remember Allah in this hour, then be among them."

Reference:
At-Tirmidhi, An-Nasa'i 1/279 and Al-Hakim. See also Al-Albani, Sahih 
At-Tirmidhi 3/183, and Jdmi'ul-'Usool with Al-Arna'ut's checking 4/144.

Chapter: Seeking forgiveness and repentance