Sunan an-Nasa'i, Volume 6, Book 51, Hadith 5722 (DarusSalam) ⓘ
It was narrated that Dawud said: "I asked Sa'eed: 'What is the drink that 'Umar bin Al-Khattab, may Allah be pleased with him, regarded as permissible?' He said: 'That which has been cooked until two-third has gone and one-third is left.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا ابْنُ أَبِي عَدِيٍّ، عَنْ دَاوُدَ، قَالَ سَأَلْتُ سَعِيدًا مَا الشَّرَابُ الَّذِي أَحَلَّهُ عُمَرُ رضى الله عنه قَالَ الَّذِي يُطْبَخُ حَتَّى يَذْهَبَ ثُلُثَاهُ وَيَبْقَى ثُلُثُهُ .
also referenced under: Hadith 5719 (Reference) & Book 51, Hadith 181 (In-Book)