Sunan an-Nasa'i 2157
It was narrated that Anas said:
"The Messenger of Allah and Zaid bin Thabit had Sahur, then they went and started to pray Subh." We said to Anas: "How long was there between their finishing and their starting to pray?" He said: "As long as it takes a man to recite fifty Verses."
أَخْبَرَنَا أَبُو الأَشْعَثِ، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، رضى الله عنه قَالَ تَسَحَّرَ رَسُولُ اللَّهِ صلى الله عليه وسلم وَزَيْدُ بْنُ ثَابِتٍ ثُمَّ قَامَا فَدَخَلاَ فِي صَلاَةِ الصُّبْحِ ‏.‏ فَقُلْنَا لأَنَسٍ كَمْ كَانَ بَيْنَ فَرَاغِهِمَا وَدُخُولِهِمَا فِي الصَّلاَةِ قَالَ قَدْرَ مَا يَقْرَأُ الإِنْسَانُ خَمْسِينَ آيَةً ‏.‏