I’ve categorized this one — “Be kind to all creatures. This is the true religion” — as fake because I haven’t been able to find anything in the scriptures that matches it.
The closest I’ve been able to find in a Buddhist source is something in a text, “The Substance of the Vinaya,” that was translated fairly early on (perhaps 70 CE) into Chinese. Here’s Samuel Beal’s 1884 account from his “Buddhism In China”:
There is, however, another ancient document containing the ethical principles of his doctrine known in China, and, so far as the present writer knows, not mentioned among southern books. This is a short work called the “Substance of the Vinaya,” which is generally printed with the little work called the “Sûtra of Forty-two Sections,” which was the first Buddhist book translated into Chinese (A. D. 70). It is probable that this is the text.referred to in the Asoka edict of Bhabra, under the title “Substance of the Vinaya”. (vinayasamākase = vinaya samākassa, contraction or summary of the Vinaya. Rhys Davids, op. cit., p. 225, n.): it is called in Chinese the “Sūtra of the Contracted Rules of the Vinaya” (lioh-shwokiau-kiai king). The same epitome of moral rules occurs as a sermon towards the end of Asvaghosha’s “Life of Buddha.” From this circumstance, taken in connexion with the other, viz., its being bound up with the “ Sûtra of Forty-two Paragraphs,” it is plain that the treatise is a primitive one, and therefore, as it bears the very name given to it in the edict, it is probably the “Substance of the Vinaya ” which was extant in Asoka’s time.
But what does the “Substance of the Vinaya” actually say?
Keep the body temperate, eat at proper times, receive no mission as a go-between, compound no philteries, abhor dissimulation, follow right doctrine, and be kind to all that lives; receive in moderation what is given, receive but hoard not up: these are, in brief, my spoken precepts. These form the groundwork of my rules; these also are the ground of ‘full emancipation.’ Enabled thus to live is rightly to receive all other things. This is true wisdom, this is the way: this code of rules hold fast and keep, and never let them slip or be destroyed.
For when pure rules of conduct are observed, then there is true religion ; without these, virtue languishes ; found yourselves, therefore, well on these my precepts.
So here we have “follow right doctrine, and be kind to all that lives,” which is not a million miles away from “Be kind to all creatures. This is the true religion.” We even have “true religion” mentioned in the phrase, “For when pure rules of conduct are observed, then there is true religion.”
But this doesn’t appear to be a canonical text — more a summary of important Vinaya (monastic discipline) rules.
The same passage is found in Ashvaghosha’s Buddhacarita, or Life of the Buddha. But again this isn’t a canonical scripture.
There are certainly lots of scriptural references to practicing kindness or compassion to all living things. For example there’s this:
As long as they live the arahants [enlightened ones] abandon and abstain from the destruction of life; with the rod and weapon laid aside, conscientious and kindly, they dwell compassionate toward all living beings. Today, for this night and day, I too shall abandon and abstain from the destruction of life; with the rod and weapon laid aside, conscientious and kindly, I too shall dwell compassionate toward all living beings.
There’s at least one passage that talks about not killing living things (which is to say, practicing compassion) is the true religion (saddhamma).
And what is the true teaching? Not killing living creatures … This is called the true teaching.
But I haven’t found anything scriptural that matches the quote in question closely.
It seems most likely to me that the source of this quote is the Hindu epic, the Mahābhārata. One of the most famous quotes from that work is “Ahimsa is the highest dharma.” Ahimsa means non-harm, kindness, or compassion, while dharma is teaching or religion. So here we have a phrase that could be paraphrased as “Be kind to all creatures. This is the true religion.”
I don’t think the Buddha would have disagreed with this. But as far as I’m aware he didn’t say it.