It is narrated from Amr bin Maymun al-Awdi (may Allah have mercy on him): I offered the Fajr prayer with Umar bin al-Khattab (may Allah be pleased with him) (at such a time) that if my son had been three cubits away from me, I would not have recognised him (because of the darkness) unless he spoke.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2146