The two halves come from different sources:
凤毛 from 《
世说新语·
容止》 (Liu Yiqing, Southern Dynasties), praising a son as having his father's 'phoenix feathers';
麟角 from 《
北史·
文苑传序》:
学者如牛毛,
成者如麟角 — 'students are as many as ox hairs, but those who succeed as rare as unicorn horns.'