“Let every man divide his money into three parts, and invest a third in land, a third in business and a third let him keep by him in reserve.” - Talmud
I was curious about this, so I tracked down the actual quote in the Talmud. The "land" in question basically means farming. I don't think we'd find that relevant today. And the talmud actually advises 0% to land / farming as the majority position.
----
173. A hundred Zuz [invested] in business, and every day meat and wine; a hundred Zuz [invested] in land, and salt and vegetables (Jeb. 63a; D. 463).
The Jews seem at one time to have had a disinclination to acquire much land, possibly on account of the uncertainty of tenure in the time of persecution. Cf. the wording of proverb no. 130. The opinions on the question of landed property differ very widely. Ben Sira says: "Hate not laborious work, neither husbandry, which the Most High hath ordained" (Ecclus. vii. 15). The fact that "husbandry" is specially mentioned is in keeping with Ben Sira's general view, shared by the Greeks, that occupation with the soil led to boorishness. A Rabbi of the second century AḌ. gives it as his opinion that there could be no worse occupation than agriculture, and on seeing a field ploughed across its breadth he exclaimed sarcastically, "Plough it also long-wise, and still you will find that to engage in commerce is more profitable" (Jeb. 63a). Rab, who lived in the third century, noticed the ears of corn being fanned by the breeze, and declared "However much you may fan, it is better to devote oneself to commerce" (ibid.). On the other hand, it is also said, "A man who does not possess a piece of land is not fit to be called a man" (ibid.); and another Rabbi adopts a middle course by advising "Let every man divide his money into three parts, and invest a third in land, a third in business, and a third let him keep by him in reserve" (B. M. 42a).
This is actually a good advice. Just remember to rebalance (not too often; in my opinion once a year is a good frequency) to buy more of what is low and sell some of what is high.
I'd phrase that more specific by saying "by selling what is too high". If something is much higher than when you bought it, but the reasons you had for buying it in the first place are still valid, then you should keep it.