“Moreover, brethren, I do not want you to be unaware that all our fathers were under the cloud, all passed through the sea, all were baptized into Moses in the cloud and in the sea, all ate the same spiritual food, and all drank the same spiritual drink. For they drank of that spiritual Rock that followed them, and that Rock was Christ.” – 1 Corinthians 10:1-4
“The well that had been given as a present to them climbed up with them on the high mountains and from the high mountains came down with them into the valleys, surrounded the entire camp of Israel, and refreshed them, each at the door of his tent.” – Targum Pseudo-Jonathan on Numbers 20:19
St. Paul here cites a well-attested and ancient Jewish tradition that the rock which Moses struck in the desert that brought forth water (see Exodus 17:1-7 and Numbers 20:2-13) followed the Israelites around the desert during their years there. No mention is made in the Old Testament of the rock following the Israelites (although, as you can see from the two Old Testament passages here, Moses brings forth water from a rock on two different occasions), but there is abundant mention of it in Jewish extra-biblical writings. Interestingly, many of these writings also identify the well of Numbers 21:16-18 with this same rock.
Although the rock is only mentioned in verse 4 of this quote, I’ve included verses 1-4 in my quote above so that the reader can see that St. Paul mentions the rock following them in a longer list of events, all of the others of which are recorded in the Old Testament. He makes no distinction between the events recorded in Scripture and the event recorded in tradition.