Imam Malik's Muwatta, Chapter: Prayer in Ramadan

Yahya related to me from Malik from Ibn Shihab from Abu Salama ibn Abd ar-Rahman ibn Awf from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, used to exhort people to watch the night in prayer in Ramadan but never ordered it definitely. He used to say, "Whoever watches the night in prayer in Ramadan with trust and expectancy, will be forgiven all his previous wrong actions." Ibn Shihab said, "The Messenger of Allah, may Allah bless him and grant him peace, died while that was still the custom, and it continued to be the custom in the khalifate of Abu Bakr and at the beginning of the khalifate of Umar ibn al-Khattab."