First of all, this verse is found with a slight variation in this chapter of a Buddhist text called the Karandavyuha Sutra:
akasam lingam ity ahuh prithivi tasya pithika |
alayah sarvabhutanam liyanal lingam uchyate ||
It is said: the sky is his liṅga,
The earth is his seat.
He is the foundation of all beings.
The liṅga is so called because they dissolve into it.
The verse you're referring to, though, has "sarvadevanam" or "all gods" rather than "sarvabhutanam" or "all created beings". In any case, concerning the "sarvadevanam" verse, in this excerpt from his book "The Origin of Om Manipadme Hum", Alexander Studholme cites other authors who claim that the verse is from the Skanda Purana, but says that he was not able to find it himself:
With regard to the couplet, it is, to say the least, unfortunate that Regamey was unable to give a precise reference for this verse in the Skanda Purana, having come across it not in any edition of the text itself, but in a modern anthology of Indian religious scriptures compiled by Alain Danielou. Danielle notes only that it appears in the Skanda Purana. The task of pinpointing the two lines is made somewhat difficult, not only because of the vast size of this purana, but also because it exists in several different editions. Like Regamey, I, too, have failed to discover the exact location of the couplet.
I've found many other books that claim this verse is from the Skanda Puranas, like this one and this one, but none that specify its location. As Studholme suggests, this may be a verse that is found in some recensions of the Skanda Purana and not others.