It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) and Sayyiduna Zayd bin Thabit (may Allah be pleased with him) ate the pre-dawn meal (suhur). When he (peace and blessings be upon him) had finished the suhur, the Messenger of Allah (peace and blessings be upon him) stood up for the prayer and prayed. (Qatadah said:) I asked Anas (may Allah be pleased with him): How long was there between their finishing the suhur and their beginning the prayer? That is, how long after finishing the suhur did he begin the prayer? Anas (may Allah be pleased with him) said: As long as it takes a man to recite fifty verses.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2141