Sunan an-Nasa'i, Hadith 5725 ⓘ
It was narrated that Bushair bin Al-Muhajir said: "I asked Al-Hasan about juice that has been cooked. He said: 'That which has been cooked until two-third of it has gone and one-third is left.'"
أَخْبَرَنَا سُوَيْدٌ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنْ بَشِيرِ بْنِ الْمُهَاجِرِ، قَالَ سَأَلْتُ الْحَسَنَ عَمَّا يُطْبَخُ مِنَ الْعَصِيرِ قَالَ مَا تَطْبُخُهُ حَتَّى يَذْهَبَ الثُّلُثَانِ وَيَبْقَى الثُّلُثُ .
also referenced under: Book 51, Hadith 187 (In-Book) & Volume 6, Book 51, Hadith 5728 (DarusSalam)