Reference : Hisn al-Muslim 252

.وَقَالَ صلى الله عليه وسلم: أَقْرَبُ مَا يَكُونُ الْعَبْدُ مِنْ رَبِّهِ وَهُوَ سَاجِدٌ فَأَكثِرُوا الدُّعَاءَ.
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Allah's Messenger (ﷺ) said: "The closest that the slave comes to his Lord is when he is prostrating, so invoke Allah much (in prostration)."

Reference:
Muslim 1/350.

Chapter: Seeking forgiveness and repentance