مُوَطَّأُ مَالِكٍ
Muwatta Malik 251
Prayer in Ramadan
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَزِيدَ بْنِ رُومَانَ، أَنَّهُ قَالَ كَانَ النَّاسُ يَقُومُونَ فِي زَمَانِ عُمَرَ بْنِ الْخَطَّابِ فِي رَمَضَانَ بِثَلاَثٍ وَعِشْرِينَ رَكْعَةً .
Yahya related to me from Malik that Yazid ibn Ruman said, "The people used to watch the night in prayer during Ramadan for twenty- three rakas in the time of Umar ibn al-Khattab."
Arabic/English book reference: Book 6, Hadith 5