釋氏要覽 2.39

2.39

即兩膝齊著地。亦先下右膝為禮○神足無極經云。月天子即從座起更整衣服。前下右膝。叉手長跪○毘奈耶云。尼女體弱。互跪要倒。佛聽長跪。

That is, both knees touch the ground together. One also lowers the right knee first to pay respect. The Scripture of the Limitless Spiritual Footsteps says: The Moon Deva Son immediately rose from his seat, readjusted his robes, lowered his right knee forward, clasped his hands, and knelt long. The Vinaya says: Nuns are physically weak; alternating kneeling will inevitably cause them to fall over. The Buddha allowed them to kneel long.