Sunan an-Nasa'i, Book 51, Hadith 185 (In-Book) ⓘ
It was narrated that Sa'eed bin Al-Musayyab said: "When At-Tila' (thickened grape juice) has been cooked and reduced to one-third, then there is nothing wrong with it."
أَخْبَرَنَا أَحْمَدُ بْنُ خَالِدٍ، عَنْ مَعْنٍ، قَالَ حَدَّثَنَا مُعَاوِيَةُ بْنُ صَالِحٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، قَالَ إِذَا طُبِخَ الطِّلاَءُ عَلَى الثُّلُثِ فَلاَ بَأْسَ بِهِ .
also referenced under: Hadith 5723 (Reference) & Volume 6, Book 51, Hadith 5726 (DarusSalam)