The Messenger of Allah ﷺ said at the time of suhoor, "Anas, I intend to fast, so bring me something to eat."I brought him some dates and a vessel of water, and this was after Bilal had given the call to prayer.He said, "Anas, find someone to eat with me."So I called Zayd bin Thabit. He came and said, "I have already drunk a cup of sawiq, and I intend to fast."The Messenger of Allah ﷺ said, "I too intend to fast."So he ate suhoor with him, then stood and prayed two rakahs, and then went out to the prayer.
قال رسول الله ﷺ وذلك عند السحور: يا أنس، إني أريد الصيام أطعمني شيئا، فأتيته بتمر وإناء فيه ماء، وذلك بعد ما أذن بلال. فقال: يا أنس انظر رجلا يأكل معي فدعوت زيد بن ثابت، فجاء فقال: إني قد شربت شربة سويق، وأنا أريد الصيام. فقال رسول الله ﷺ: وأنا أريد الصيام فتسحر معه، ثم قام فصلى ركعتين، ثم خرج إلى الصلاة.
Nasai (2167) narrated it via Abdul-Razzaq, and it is in his Musannaf (7605). Imam Ahmad (13033) narrated from him, from Ma'mar, from Qatada, from Anas, who related it. Its chain of transmission is sahih.His statement, "and that was after Bilal had given the call to prayer," refers to the first adhan, which takes place at night for the tahajjud prayer.