The Messenger of Allah ﷺ and Zayd bin Thabit ate their predawn meal together. When they finished, the Messenger of Allah ﷺ rose to perform the prayer.We asked Anas, "How long was it between their finishing the meal and their entering into the prayer?"He replied, "About the time it takes a man to recite fifty verses."
حدثنا محمد بن جعفر حدثنا سعيد بن أبي عروبة عن قتادة
أن رسول الله ﷺ وزيد بن ثابت تسحرا فلما فرغا من سحورهما قام رسول الله ﷺ إلى الصلاة فصلى فقلنا لأنس كم كان بين فراغهما وسحورهما ودخولهما في الصلاة قال كان قدر ما يقرأ رجل خمسين آية
Its isnad is sahih. It is found in Bukhari 3/18, no. 1134 (Fath), in the chapter on tahajjud, on one who ate the pre-dawn meal and then did not sleep. And in Nasai 4/143, no. 2155, in the chapter on fasting, on the length of time between the pre-dawn meal and dawn.