Zaid bin Thabit said: "We ate Sahar with the Messenger of Allah, then we stood for the Salat." I (Anas) said: "How long was that?" He said: "About the lengthy of fifty Ayahs."  

تَسَحَّرْنَا مَعَ النَّبِيِّ ﷺ ثُمَّ قُمْنَا إِلَى الصَّلاَةِ قَالَ قُلْتُ كَمْ كَانَ قَدْرُ ذَلِكَ قَالَ قَدْرُ خَمْسِينَ آيَةً  

(Another chain) except that he said: "About the length for reciting fifty Ayahs."