瑜伽論記 10.479

10.479

第一難行者，謂菩薩已得引發神通，極善成熟靜慮應生上界，為利有情故還生欲界，是為難行，此中意也。景云：此不起煩惱，但種子潤生起或潤生此亦無損，以七地已來皆有利益，須起煩惱爾時即起。

As for the first difficult practice, it refers to a bodhisattva who has already attained the elicitation of supernormal knowledges and has extremely well mastered dhyāna, who ought to be reborn in the higher realms, but for the sake of benefiting sentient beings is reborn back into the desire realm. This is considered a difficult practice; this is the meaning here. Jing says: Here, afflictions are not aroused; rather, they are merely moisturized and produced by seeds, or [even if] afflictions moisturize birth, this causes no harm, because up to the seventh stage there is always benefit, and when afflictions need to be aroused, they are aroused then.