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Al-Adab al-Mufrad 623
حَدَّثَنَا عَبْدُ اللهِ بْنُ يَزِيدَ، قَالَ: حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ زِيَادٍ قَالَ لِي عَبْدُ اللهِ بْنُ يَزِيدَ: سَمِعْتُ عَبْدَ اللهِ بْنَ عَمْرٍو، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ: أَسْرَعُ الدُّعَاءِ إِجَابَةً دُعَاءُ غَائِبٍ لِغَائِبٍ.
'Abdullah ibn 'Amr reported that the Prophet, may Allah bless him and grant him peace, said, "The swiftest supplication to be answered is the supplication of someone for another person who is not present."
Cited in Al-Adab al-Mufrad (No. 623).