Mukhtasar Sahih Bukhari — Hadith 955

Narrated from Sayyiduna Abu Sa'id Khudri, may Allah be pleased with him, that he heard the Messenger of Allah, peace and blessings be upon him, saying: "(O people!) Do not observe continuous fasting (without Suhoor and Iftar), and if any of you insists on doing so, then he may do so only until Suhoor."