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كتاب مواقيت الصلاة
9
Times of the Prayers
(27)
Chapter: Time of the Fajr (early morning) prayer
(27)
باب وَقْتِ الْفَجْرِ
Sahih al-Bukhari 575

Narrated Anas:

Zaid bin Thabit said, "We took the "Suhur" (the meal taken before dawn while fasting is observed) with the Prophet (ﷺ) and then stood up for the (morning) prayer." I asked him how long the interval between the two (Suhur and prayer) was. He replied, 'The interval between the two was just sufficient to recite fifty to Sixth 'Ayat."

حَدَّثَنَا عَمْرُو بْنُ عَاصِمٍ، قَالَ حَدَّثَنَا هَمَّامٌ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ زَيْدَ بْنَ ثَابِتٍ، حَدَّثَهُ أَنَّهُمْ، تَسَحَّرُوا مَعَ النَّبِيِّ صلى الله عليه وسلم ثُمَّ قَامُوا إِلَى الصَّلاَةِ‏.‏ قُلْتُ كَمْ بَيْنَهُمَا قَالَ قَدْرُ خَمْسِينَ أَوْ سِتِّينَ ـ يَعْنِي آيَةً ـ ح‏.‏
Reference : Sahih al-Bukhari 575
In-book reference : Book 9, Hadith 51
USC-MSA web (English) reference : Vol. 1, Book 10, Hadith 549
  (deprecated numbering scheme)