This is a saying from the Pali canon, upadhi dukkhassa mūlanti, which means “Attachment is the root of suffering.” So this is a genuine canonical quote.
You’ll find it in this sutta, but translated by Thanissaro as “Acquisition is the root of stress.” His translations are rather idiosyncratic.
In this translation of the same sutta it’s “acquisition is the root of suffering.”
Bhikkhu Bodhi’s translation (not available online, but in The Middle Length Discourses of the Buddha, page 868) has “attachment is the root of suffering,” although he sometimes has “acquisition” in place of “attachment,” in various repetitions of the phrase.
“Whatever is not yours: let go of it. Your letting go of it will be for your long-term happiness and benefit” is a genuine quote from the Buddhist scriptures. It’s from the Na Tumhaka Sutta of the Samyutta Nikaya.
“Whatever is not yours … your letting go of it will be for your longterm happiness & benefit.” Buddha
“Whatever living beings there may be — feeble or strong, long, stout, or of medium size, short, small, large, those seen or those unseen, those dwelling far or near, those who are born as well as those yet to be born — may all beings have happy minds.”
—The Buddha, Karaniya Metta Sutta
“Having gone on his almsround, the sage should then go to the forest, standing or taking a seat at the foot of a tree. The enlightened one, intent on jhana, should find delight in the forest, should practice jhana at the foot of a tree, attaining his own satisfaction.” The Buddha
The Buddha. (Source: Nalaka Sutta, Sutta Nipata.)