Imam Al-Dardir and Al-Dusuqi mention this in Hashiyat Al-Dusuqi.
It is when the light of the moon is noticeably reduced- whether the moon is totally covered or partially. If the light is disrupted negligibly, then the prayer is not recommended yet.
Written by Uthman Qureshi
Source: Hashiyat Al-Dusuqi ala Al-Sharh Al-Kabir, Dar ul Fikr, vol 1, pp 402.
وَأَصْلُ النَّدْبِ يَحْصُلُ بِرَكْعَتَيْنِ وَمَا زَادَ فَمَنْدُوبٌ آخَرُ (لِخُسُوفِ قَمَرٍ) أَيْ لِذَهَابِ ضَوْئِهِ أَوْ بَعْضِهِ
قَوْلُهُ أَيْ لِذَهَابِ ضَوْئِهِ أَوْ بَعْضِهِ) أَيْ مَا لَمْ يَقِلَّ الذَّاهِبُ جِدًّا وَإِلَّا لَمْ يَصِلْ لِذَلِكَ
