Matsunaga Yūkei 松長有慶 (b. 1929), a scholar of Shingon Buddhism and up until recently the chief at Kōyasan, in 1966 published an article (see here in Japanese) that pointed out the existence of some Sanskrit fragments of the Mahāvairocana-sūtra 大日經. Although no extant Sanskrit version of the text is known to exist, one fragment from the text is found in a citation in the Bhāvanā-krama by Kamalaśīla 蓮華戒 (fl. 8th century). It is the core phrase of the Mahāvairocana-sūtra which in Chinese reads as follows:
「菩提心為因。悲為根本。方便為究竟。」(CBETA, T18, no. 848, p. 1, b29-c1)
Bodhicitta is the cause, compassion is the root, and expedient means (upāya) are the conclusion.
The Sanskrit fragment in the Bhāvanā-krama reads:
tad etat sarvajñānaṃ karuṇāmūlaṃ bodhicittahetukam upāyaparyavasānam iti |
As Matsunaga points out, the first two phrases are reversed: in the Sanskrit karuṇā is mentioned before bodhicitta. The Song-era translation of the Bhāvanā-krama by Dānapāla 施護 (d. 1017) follows the order of the Sanskrit:
「如毘盧遮那成佛經說。所有一切智智。悲心為根本。從悲發生大菩提心。然後起諸方便。」(CBETA, T32, no. 1664, p. 565, b8-10)
As the *Vairocanābhisaṃbodhi-sūtra states, ‘Omniscience: compassion is the root; great bodhicitta is produced from compassion, thereafter giving rise to expedient means.’
Matsunaga suggests this shift to citing compassion first reflects the tendency of the Bhāvanā-krama to focus on compassion (the introduction states that compassion comes before bodhicitta). This stands in contrast to the original Mahāvairocana-sūtra which is oriented around a Madhyamaka framework with an emphasis on śūnyatā. The Chinese commentary also reflects this understanding (此菩提心為後二句因).
In light of that, the Sanskrit fragment is perhaps not a direct quotation, but rather a paraphrasing of the original.