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Al-Adab al-Mufrad 1170
وَحَدَّثَنِي أَبُو صَالِحٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم مِثْلَهُ، قُلْنَا: فَإِنْ كَانُوا أَرْبَعَةً؟ قَالَ: لا يَضُرُّهُ.
Ibn 'Umar reported something similar from the Prophet, may Allah bless him and grant him peace. He stated, "We said, 'If there are four?' He said, 'Then it will not harm him.'"
Cited in Al-Adab al-Mufrad (No. 1170).