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Hisn al-Muslim (Fortress of the Muslim) 252
وَقَالَ صلى الله عليه وسلم: ((أَقْرَبُ مَا يَكُونُ الْعَبْدُ مِنْ رَبِّهِ وَهُوَ سَاجِدٌ فَأَكثِرُوا الدُّعَاءَ)).
(He also said: ‘The nearest a servant is to his Lord is when he is prostrating, so supplicate much therein.)
Cited in Hisn al-Muslim (Fortress of the Muslim) (No. 252).