ENOfficial
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."
Source: sunnah.com
Other translations
Arabic
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 .
sunnah.com