Ishaq bin Mansur al-Kausaj إسحاق بن منصور بن بهرام - الكوسج · The Book of Fasting

"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."

"Some food was brought to the Messenger of Allah at Marr Az-Zahran, and he said to Abu Bakr and 'Umar: 'Come and eat.' They said: 'We are fasting.' He said: 'Saddle the camels for your companions, and help your companions."'(Daif)