Reproduced from A.R. Nykl, "Arabic Inscriptions in Portugal," Ars Islamica 11/12 (1946): 182.
The Arabic text: الدائم الّله \ترحّم\بفضلك يا\ واقفا و انظر\ مكانا دفعة\ اللّه محمّد. Nykl’s translation of the inscription is as follows: "The Eternal is God only. Say ‘may God have mercy upon him,’ you who are stopping [in front of the stone] and contemplate the place for a while. Allah… Muhammad."