Hadith 502510
Chapter 6: Prayer in Ramadan - كتاب الصلاة فى رمضان
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."
Reference Muwatta Malik 502510
USC-MSA web (English) reference : Book 6, Hadith 5