وَقَالَ أَبُو يُوسُفَ وَمُحَمَّدٌ رَحِمَهُمَا اللَّهُ: لَا شَيْءَ فِي الزِّيَادَةِ حَتَّى تَبْلُغَ سِتِّينَ، فَيَكُونُ فِيهَا: تَبِيعَانِ أَوْ تَبِيعَتَانِ،
Translation: Abu Yusuf and Muhammad say that nothing is due on the increase until sixty; at sixty, two tabi‘ or two tabi‘ah are due.
Abu Yusuf and Muhammad hold that the increase above 40 is not separately assessed until the herd reaches 60. Thus 41, 50 and 59 retain the due musinn/musinnah of the 40 bracket under their view.
At 60, the obligation becomes two tabi‘ or two tabi‘ah. The source commentary also reports this view as a narration from Abu Hanifah.